When I was looking at used GN trailers, I had the problem of pickup bed clearance.  The 4x4 pickup trucks have a higher bed; the Ford new body style (2004 and newer) has a deep bed too. Thereby reducing the amount of space between the top of the sides of the bed and the bottom of the GN area.

Otherwise, you are on the right track for all the things to check for. a 10 year old trailer is NOT that old. The Hawk brand is very good and so long as it was cared for, should have many more years of service. I would look for things that showed me the previous owners took care of the trailer. I'm more concerned about wheel bearings, axles, hinges, ramp attachments (if the trailer has one), lights working (I hate trying to fix electrical shorts), etc. etc.  A/C can be replaced and if it's not working, I can still get safely down the road. plumbing leaks. Again, they can be fixed - but if they existed - what damage have they caused?  mouse infestations - yuck - but you can usually SMELL it if they are in there.

If the tires are more than 4-5 years old, then strong consideration to replace them should be given due to rot and possible cracking of the sidewall. But it's a matter of actually examining the tires and determining if they NEED to be replaced or not.

I would be concerned about the entire floor - in the horse area AND the LQ. The horse area floor is more likely to have problems than the LQ - so checking the floor there would be a higher priority for me. I don't see why the LQ floor would be a problem unless there had been a roof or plumbing leak.

I say - look it over, check it out, compare it to other similar trailers, make sure it's compatible with the tow vehicle - weight, clearance, etc.

Another thing to consider, is the proper working condition of all the appliances in the LQ; furnace, fridge, water heater, stove, air conditioner, vent fans, toilet flushing, faucets etc. An external inspection of the trailer should include an examination of the undercarriage for cracks, bended framing, damaged flooring, loose or missing plumbing etc. If possible have the wheels pulled and have the brakes inspected.

The plumbing of a trailer is important, and should be thoroughly checked for proper operation and leaks. With water in the storage tank and the pump on, all the faucets off, the pump should not continuously run or cycle often. If it does you probably have a leak, and by looking under the trailer, you may see water escaping. Some LQ installers, have supply lines installed between the layers of the flooring. If any leaks are within this area, repairs can be both expensive and time consuming. The external holding tanks, gray and black, as well as their associated plumbing, are subject to damage, and should be checked for leaks after running the toilet for several cycles.

The roof area should be inspected for damage, proper operation of the stall vents, damage to the cooling fins of the air conditioner, external plumbing and shower vents.

All the doors, windows and stall dividers should be inspected for proper operation, with the hinges and latches being especially noted. All the doors and window should close easily without slamming and should sit squarely within their frames. None should sag or chafe one area of the frame surround.

If the tires appear to be in good shape, the build dates can be checked on the side walls. They would only need to be replaced if they exceed 6 years or older, or if they show damage. The wheels should be checked for rim damage, including bends or cracks.

The propane tanks may also have to be updated or replaced, to include the new style valves. The safety equipment will probably have to be updated with a new fire extinguisher, CO and smoke detector. The latter usually have a five year life span.

A good used trailer is an excellent value. Hope your friend buys well and enjoys the "new trailer".

Gard

 Besides all the other items mentioned:

Take a ladder and inspect the roof - especially the caulking as it may need to be replaced and the LQ could be leaking if there are cracks. 

Not sure if this was mentioned - but check the brakes.  I have an '03 and when I took it in this year for the wheel bearings to be repacked, the company also does a safety check.  I needed all four brakes replaced.

As mentioned - check every appliance to make sure they do work - do not take their word for it as they might not have checked everything in awhile.  

I have friends, who, this year.... both purchased older aluminum 3h, LQ's.  One is a 95  and one a 97,  by coincidence both are Sundowner. 

 One came from TX, and one from AR.. One friend, as a precaution, had th roof caulking cleaned off and redone.  The other needed one new breaker put in the breaker box, and some running lights replaced and a new plug to change the 6 wire plug to a 7 wire to match their truck..  They both had awnings, which they unrolled and inspected before purchase, which functioned and looked fine.  One needed to tighten the fittings under the kitchen sink, which took 5 minutes, and that sums up the "repairs" that were needed. 

 Both LQ's were used but not abused, and they are all very very pleased with their purchases.  The prepurchase full roof to undercarriage inspection, pulling mats and checking the horse compartment floor, the run through on all appliances working, etc as others have mentioned were done.  They both pulled em home without having the wheels pulled to inspect the brakes or bearings, they had that done after they got home.....both of em had the wheels pulled and bearings repacked just to "know", one needed the brakes adjusted. Both had bearings in good shape.  They are very pleased with their purchases and the thousands that they saved.

Good luck in your purchase, do your homework, know that some odd things may need to be done, but these types of purchases are done successfully every day, best wishes for your new to you rig!