I think your wise to buy your first one used. Most of us don't keep the first RV very long before we figure out we want after a little experience.
I would make a list of the things you really wan't. Maybe a walk around bed, seating for "X" people, etc.
Then scout the RV shows to get an idea of whats out there. When buying used look for evidence of leaks. Look for stains on the ceiling and inside of cabinets. Look for soft spots in the floor too. Use your nose, if you walk into a trailer and it smells musty then walk away - it's probably got a leak and mold. They don't smell that way just because they've been closed up.
Plan on doing maint. to the bearings and brakes of anything you buy.
Good luck and welcome to a great hobby!
Scott