There are so many options, and really depends on your budget.
My trailer is a steel 3 horse slant load with front tack room (bumper pull). I usually haul one horse, but it is great to have the option to haul more if needed. Mine is a big open door at the back with a step up - I've never had a problem loading a horse into it. I really dislike the trailers with the rear tack or small loading doors.
My trailer IS heavy and it does get some rust. I had it sandblasted and painted last year, and can already see some rust coming back. BUT, I like that we can easily do any welding repairs ourselves. And I do haul on lots of back-roads which the trailer can with stand.
I tow with a 3/4 ton diesel - no problems there.
If I had the money to upgrade, I would like to get a steel frame/aluminum skin gooseneck. 3 horse slant with front dressing room (and air ride).
BUT, the one i have now suits my needs and is actually a really great trailer. It is my first trailer, so beggars can't be choosers.
Aside from choosing the type of trailer, please be sure to check the floor boards, brakes, tires, bearings, etc. I had a small electrical problem with my trailer - and I won't even get into how frustrating that was to figure out!