I am currently trailer shopping also (looking for a 3H with 10' sw.)

I have been told the one luxury many people swear by is the hydraulic jack.

I have been told by two dealers here in Texas that there are problems with the werm flooring staying down.  The Cimarron dealer here in Beaumont won't put it in any trailer.  Maybe it is the Texas heat and humidity, because I haven't heard of any problems with the werm floor anywhere else.  It was one of those luxury items I wanted and now I will have to think about that, also.  Good luck!

A 18' long 3h doesn't have much space for a LQ.  If you are not going to build in aLQ go for the 7'6"

Absolutely go the 7'6" tall. If you ever think you'll spend a night in the Gooseneck sleeping.

Drop downs give you better ventilation. But if you don't have summer heat, save the money and do the bus windows.

Drop down windows and bars on the head side. Absolutely

One roof vent above each horse should be enough

I liked my telescoping divider on my old 3 horse.  If and when I needed to load a 4th horse, I could colaspe that divider and squeeze three horses into the space of two. allowing me to haul 4 horses in a bind. With a rear tack you loose this.  But I really liked it on my old 3 horse.

Get all the lights you can get. I use mine more for camping. I'll turn them on when I'm backing up.  Never can have too much light.

I like the Rumber flooring. No Opinion on WERM.

Saddle rack is your preference.  I never move mine

Can't get too many blanket bars

From my way of thinking and experience... I do trail riding and camping.  For a trailer that serves long and well but first cost is a consideration get the options that are costly to add later.

extra height-sleeping in the GN area is cramped enough.  The tall bulkhead for the new tall trucks takes away from the headroom.

8' width takes away the running board but a step can be added later.  use a mounting step at first

A pass though door between the DR and horse area makes the horse area useful for extra stuff.

An escape door in the first stall becomes a mud room or pantry.

An AC unit framing & supports allows a hassle free AC install later on.

The rump drop downs make the extra vents unneeded.  My trailer has drops front and rear.  No bars on the rear drops.  They make ventilation a snap in the hottest weather.

Roof insulation is good when-ever there's sunshine.  Even in cool weather, the roof will get hot to the touch.  But insulation is easily added later.

Sundowner uses some other product beside WERM but I sure like the coated horse area for clean-up.

I never have the stink'in saddles or blankets in the DR.  I removed the rack mount from the DR.

I agree, the mounting blocks can also be tippy if the ground is uneven.

   Get everything you think you may want on that trailer ...NOW! Don't skimp, it will only cost you more in the long run. Get the slide out-fold down step. Get the roof and walls lined and insulated. If you are not hauling corral panels on the side of the trailer, then by all means, make it 8' wide and 7' 6" tall. You may save a little $$$$ up front, but down the road you'll wish you had those bars to hang those blankets and jackets rather than dropping them on the floor in the poop and pee!!!! See what I'm getting at?! Sounds like it's going to be a nice trailer, congratulations!!

P.S.  Add you a few feet to that dressing room, you'll be glad you did.

Here's my take on your choice of options...for what it's worth:

Unless you have huge horses or you're carrying a carriage or golf cart, go with a 7 ft wide.  Visibility and maneuverability are less with an 8 ft and my experience says if you are towing an 8 ft wide trailer you probably should be towing with a dually - again, just my experience, I'm sure there are others who will disagree.

Definitely go with the 7'6" height.

Skip the dropdown windows on the butt side.

Go for the dropdown bars on the head side.

Get one roof vent above each stall. 

Keep the RV door with screen.

Get all the extra lights you can.

Can't comment on the werm floor.  Mine is all aluminum with rubber mats in the horse area and I've had no issues.

Skip the saddle rack mount in the DR.  I also do not want smelly tack in my DR/LQ

Extra Blanket bar - same issue

Clothes Bar is nice to have.

Insulate everything you can before you buy, especially the roof.

Fold down step is really nice.

Also consider a pass through door between the DR and the horse area.  This is a big plus!

Collapsible rear tack is a big plus! It allows you to open the rear if needed for a horse that likes more open space to load.

Make sure to get a spare tire and mount.

Of course these are only opinions - I'm on my third trailer since having my own horses.  I currently have a 2 h GN slant load Silver Star, mostly only haul one horse, but I do camp with the horse and trailer.  Most of the things that you list as options were standard equipment on my trailer.

Good luck with your purchase.

18 feet doesn't sound very big for a three horse.  Make sure you have enough space in your DR in case you need to use it for camping/sleeping at some point.

For what it's worth, here's my $.02:

+ Increase width of trailer to 8 ft. (but still thinking about 7'6")  7'6" is adequate. 
+ Increase height to 7'6 ft.  Absolutely.  The extra height will be appreciated as you climb in or out of the sleeper.
+ Dropdown windows instead of bus windows on rump side and rear load door  I don't see a big need for this.
+ Dropdown bars on all dropdown windows  Absolutley.
+ Additional roof vents in horse stalls  Again, I don't see a need.
+ RV style door with screen door instead of standard  For sure.
+ Telescope last divider I'll admit, I see this as a good option, but I've yet to retract mine.
+ Extra halogen load lights (2) Can't ever have too many.
+ WERM flooring Or Rumber.  Both are great.
+ Additional setting for removable saddle rack post in DR  Ummm, your DR is gonna be tight as is.  I'd forgo this.
+ Additional blanket bar  Sure.
+ Clothes rod  Sure, we use ours

Items that I considered but ultimately discarded as not being necessary or not worth the extra cost:

+ Fold-down step (think I'll just use a mounting block)  I guess so, if it's not a big add.  Mntg block works fine for me, but DO NOT lower trailer onto mntg block - don't ask.
+ Roof insulation (I'm in central coast Calif. where it doesn't get excessively cold or excessively hot)  Insulate, without hesitation.  Your horses will love you more. 

I'm sure they've been mentioned, but other things to consider:

Pass thru door-Escape Door-Stud wall (your fist stall will become your bucket room/bathroom/feed room/junk room once you arrive at your destination). A/C reinforcing.  Power vent in DR.  Add 2' to dr.  Wire DR for 110v.

I would add spring back dividers, side tack door with tie ring at end of trailer, (tie your horse there, pull out your saddle and throw it on) seal the tack so that shavings and dust don't get in, full to floor divider at first stall is great for using first stall for hay etc., I don't see any reason for a removable saddle rack unless you are making the rear tack collapsible, if you go with 7 1/2' tall, lower all window latches or add the little fold down step cleats so that you can reach the latches, add a couple of feet to the dressing room just in case you ever want a small LQ, it doesn't add much cost to the trailer, I own a 4-Star, nice trailer, but in my opinion doesn't compare to the quality of the Platinum that I used to own!  Last thing, get an extra set of keys!

Here's my thoughts and I am in the central valley of CA, but have hauled alot in state and out of state. 

Increase width of trailer to 8 ft. As mentioned, this is not a LQ, so 7'6 is plenty wide
+ Increase height to 7'6 ft. Probably good for resale value or if your horses are large or if going full insulation & sleeping alot in GN
+ Dropdown windows instead of bus windows on rump side and rear load door Sliding bus windows should suffice.
+ Dropdown bars on all dropdown windows Agreed, use these & keep drop downs headside
+ Additional roof vents in horse stalls Nice for resale purposes
+ RV style door with screen door instead of standard Nice extra feature (but depends on if you will actually be sleeping in this trailer). Nice for resale
+ Telescope last divider This is handy
+ Extra halogen load lights (2) You can't have too much light
+ WERM flooring I am considering it for my trailer (after market install) add resale value
+ Additional setting for removable saddle rack post in DR (Are you going to sleep in the DR or are you going to use it for a tack room?)
+ Additional blanket bar Same as saddle post
+ Clothes rod This is always handy

Items that I considered but ultimately discarded as not being necessary or not worth the extra cost:

+ Fold-down step (think I'll just use a mounting block) You will be sad if you don't
+ Roof insulation (I'm in central coast Calif. where it doesn't get excessively cold or excessively hot) You will get condensation inside on the coast if it is not insulated in the cooler months, much easier to insulate out of the box than to add later...again your choice. Would add resale value.

How am I doing here? All thoughts greatly appreciated!

Additionally, make sure they install battery power for your lights, this is a nice extra feature, I also like having a grounded plug in the DR which I can use for last minute clipping etc, it comes in handy.

Again, this is just my .02c!

Not sure how much of an "add " it would be, but considering your circumstances, have you considered going with a mid tack as an add on? You could then eliminate the rear tack and have a wider opening for horses to get in (if this is a concern). This may eliminate the need for using the DR area for a tack storage.

Also, if you do end up using the DR as your tack area, I highly suggest adding bridle hooks as well as the blanket bars. You may also think about adding additional brush boxes as well. A boot box might also be good for storage and allow for easy access to the GN area for sleeping, etc. Since you may also be using the DR for tack storage, I would also suggest the step going in as you will be going in and out alot.

I went from one trailer that had a rear tack ( New Cherokee) to another that does not (Used Jamco) and only has tack storage in the DR area and I can tell you, I really missed that rear tack and if I had to do it again, I would opt for a mid tack area. I also added a step up from Coolhorse, it was welded on and folds up when not in use and it is so nice to have now that the only tack storage is in the DR area.

You will love the new 4 Star!

I am also close to buying a 4-star and my dealer has said that the extra height is needed and insisits on 5200 lb axels (with heavier brakes etc.) vs the std 3200 lb.  Also looking at the WERM flooring, but want to knwo if there have been any problems.  I didn;t like the drop down windows.  Think load lights are important.  I don't want the extra width.

My 2 cents !