Lhotka, lots of questions need to be answered prior to the big purchase, things such as: Gooseneck or bumper pull,
aluminum or steel (pros and cons for both), slant load or streight, type floor; treated wood, lifetime rubber composit, aluminum, living quarters or not. And the list of things to consider goes on.
I'm in the same boat as you, need to buy a another trailer, will likely be either a used aluminum or a new steel, 3 horse slant with a 4' to 7' short wall in the dressing room (will convert to living area) with the entry door on the left side, if it dosen't have a pass though door into the back, I'll install one, ideally will have drop down windows, with removable rear tack.
On things to look for on a used trailer. Used trailers are kinda like used cars/trucks, a bargan rarely is, by the time you fix up a fixer-upper, you could have bought a better trailer for the same or less money and a lot less trouble (ask me how I know).
Check for the obvious, rust, missing or corroded frame members, if there is a rubber mat, pull it up and check the floor carefully, on wood floors, stomp around looking for soft spots, use a screw driver to probe suspect areas. On aluminum floors, look for discolored (white) areas, could be a sign of aluminum oxidation, if the owners didn't remove mats and wash the urine out often, the floor could be weakend to the point of being dangerious.
Best advise, go to several dealers, ask questions, tellem up front that you're gonna buy soon, just not today, and you want to get as knowledgeable as possible before spending your money. If possible, carry someone you trust who knows trailers to go with you and act as your advisor.
Good luck, Ronnie