We are on trailer #2--both steel, 4 horse. The next one will be aluminum, but NOT because we can't pull it--we haul a gooseneck, 4 horse slant-load with a 2007 Dodge full ton Cummins diesel doolie. We're just tired of the rust and find ourselves spending more time sleeping in the front overhang.
I was going to mention check out the floor, but I guess you did. Our first trailer did NOT have a sealed floor. We pulled up the steel lip, and pulled out and put in our OWN floor after only 3 years. I think we sealed it, like 4 times!! Also, we had overloaded our first trailer, and stock 4 horse, straight load and damaged the axles. Then, while travelling out west one of the axles was bending, and our tires started smoking. The garage where we stopped couldn't fix the axles, so they pared off the steel that was rubbing against the tires. We got to the Black Hills, SD, dropped off our horses in camp, and drove the trailer back to Rapid City to a horse trailer place to repair. They replaced the axles and put heavy duty semi-type shocks on--GREAT ride after that! (Only charged us $300.00 for that, some 20 years ago. They wouldn't take more--I was SO grateful!!)
IF your trailer doesn't have mats, you can put it stall mats--you know, the cattle mats with the buttons, so you actually take them OUT again. My horses travelled much quieter after I went from small mats that JUST fit them, to mats which completely covered the floor.
Best buy for a trailer is a stock trailer, like the one you looked at. They are great to use as a moving van, too!! If you're worried about dust and debris, put fly masks on your horses.
Bumper hitch trailers follow better with the correct hitch. Ours had sway bars, but find a lot that specializes in horse trailers and they can recommend what's best and least expensive for you.
What they DON'T tell you when you buy is what happens after you blow ONE tire and are riding on the spare. We have TWO spares, one on each side. And, yes, we have had to use both.
Buy a Trailer-AId
http://www.trailer-parts-forless.com/trailer_aid_jack.htm
and have it LIVE in you tack room. It is definitely the safest jack because you roll the other tire up onto it and it holds the entire weight of horses and trailer safely.
WRAP YOUR HORSES LEGS!!!!!! (At LEAST, consider using shipping boots that cover the entire cannon bone AND pasterns.) I have seen firsthand what happens without a wrap. Fortunately, the horse who was injured did it unloading at the vet, but the skin was ripped almost all of the way to the bone. I have used cheap quilts and the cheapest polo wraps. Much better to replace those than permanently lame your good horse.
If your horse is very tall, consider a head bumper, as well.
Do NOT ever load your horse without attaching the halter to a trailer tie--WITH a quick release. He could be nibbling from the floor and you hit the brakes and he can't balance himself--any NUMBER of different injuries could occur.
If you get a trailer with windows that open out DO NOT leave the windows open while you travel. Remember the old saying (for kids): "Don't stick your elbow out too far---it might go home in another car!"....yeah
We had to learn ALL of our trailer lessons the hard way, that's why I've shared.