I've got a two horse slant load - the horses and I love it. I don't have a back tackroom so there is plenty of room to get the horses in and out. I do have a walk in tackroom in the front. It's also got some nice extra features like interior lighting, rubber bumper, etc. It's size is 7 ft high and I'm not remembering the width right now but it's not extremely wide. My biggest horse is 15 hands and fits very nicely in the trailer.
With the old trailers, I think you need to watch for structural integrity (good flooring, no rusting out spots, good trailer hitch and axles, etc), working brakes, working lighting system (good ground, no exposed spots, etc) and straight pulling. Also, if it's an "enclosed" trailer, you'll want a ventilation system and no drafts. You may find that trailer a bit heavier if it is all steel construction as well. After those areas are checked, it's just might be worth purchasing.