I would go for 7 feet. I know my 16.3hh Percheron can only be trailered in a 7ft+ tall trailer comfortably. Rust is a problem if it's in the bottom in all the mechanics of the trailer. Make sure the breaks work, make sure it really is in good condition and new paint doesn't just mask a crappy trailer. If the floors are bad, they can be replaced, but good floor boards are a must for trailering.
Check everything. ESPECIALLY breaks and floor, then lights (although those usually are easy to fix), how easy it is to connect it to the truck (as in some it's rusted so badly you can't even lock it in place), if all locks on doors are smooth and working.
I'm not sure about the height, but when I was getting mine (I got new although the cheapest one on market, Valley stock), I went with extra-tall rather than extra-wide. My neighbor used mine to haul her 16'2 OTTB and he fit in perfectly.
One thing is if you're purchasing locally, then you can always ask the owner of the trailer if you can put your horse in it to make sure he fits. You'll want to take it for a "test drive" anyway.
Rust is only a problem if it's on supports. Even brakes will rust a bit, but once applied a few times it should be all gone. If the trailer brakes are grabby past the first 2 or 3 stops on your test drive, then something more serious is likely wrong with them.
I know it's a pain, but if the floors are matted, you need to pick them up and check the wood planks