Lots of questions. I see you are in Mexican Hat, So the close dealers are going to be in Cortez or Grand Junction.  Even they aer a ways off, but they are porbably who you will have to go to for service. What brands do the dealers in those towns offer? 

Are you going to buy New or Used?

With a Ford F250 V10 you are limited to about 13,500 lbs of trailer with a tongue weight less than 2900 lbs.  You will have to keep that in mind.

I had a Classic Bunk House trailer that I sold a couple of years ago that weighed about 13,000 when loaded. So that might be a consideration. That's not a full blown living quarters, but it did have a bathroom, shower, couch and bed.  Frontier Trailer in Spanish Fork sells that brand. It's an all aluminum trailer but not in the same class as a Platinum or 4 Star as far as top of the line construction.  But mine held up very well.

There are several Logan Coach dealers around, St George, Cedar City, Salt Lake, Grand Junction. Their Gold or Platinum LQ in their XT or XTR line of trailers will put you pretty close to max for your 3/4 truck with a 2 horse and over with a 3 or 4 horse trailer.

Just know that most gooseneck horse trailers are going to weigh about 6,000 lbs empty.  Add your LQ and horses and you will be 13,000 to 16,000lbs real quick.

I pull with a 99 Chevy with 6.0L gas engine...it's a 4x4 with extra cab, Shortbox.  My GN is a 2H Kiefer Built with 9' SW with shower, bathroom, furnace, pull out sofa and small living space.  It weighs 5300 empty; that is with no water, mats, horses, etc.  It is about the same size inside as my friend's 10' camper - okay for one or two people.  I ride endurance and camp in the boondocks, a lot.  The trailer has 2 propane tanks and 2 big marine batteries which are great for weekends.  For longer stays I have a Honda 3000 generator to help revive the batteries!   Last spring I saw a trailer that was very cool...  it was a Classic 2H GN with a slide-out in the living room area.  That made an amazing difference in the amount of space you have to move around.  I have trailer envy for sure!  She was pulling with a 3/4 truck as well.  I think she said the weight was 7000 lbs so probably 9000 loaded with her stuff and 1 horse.  It was a nice trailer.

Did you and your friend weigh these trailers or is this what the marketing literature tells you the trailers weigh?

5300 lbs empty sounds like the weight before LQ to me. 

Just a suggestion... The truck stops have scales that give three weights at once.  steer axle, drive axle and trailer axles.  I would wait to have a full load aboard the trailer.  Get the weights of all axles.  Then go home and let the horses off, unhitch the trailer and just drive the truck back to any Scale.  You'll get the drive and steer axle weights again. 

The difference between the sum of steer & drive axle weights with and without the trailer is your hitch pin weight.  The difference between the sum of the truck axles without the trailer and the sum of all the axles (Include the trailer axles)again with the trailer is the trailer weight.

The advantage is: not having an unhitched trailer full of horses sitting in the parking lot of the truck stop.

i currently have a merhow 'very lite' (with 'weekend' package, 6' SW.) and am looking to go bigger. (2 kids and a mom just can't all fit in a weekender, esp when one kid is a teenager.)

the merhow is lite, easy to pull and fairly well built but i don't know if it would hold up to a lot of rough roads, like those in some campgrounds, off the beaten path type thing.  

spend a lot of time looking on this site at the various trailers in your pricerange.  it is going to be tough going from an RV to  a small LQs.  have you thought about getting an RV to pull a 2-horse?  i have seen people do that, too.

i have also taken a piece of chalk and 'drawn' out the living quarters, the floor plan, according to dimensions.  helps to get an idea of how crowded things can get.     good luck.