Pick the floor plan that fits you the best. Go live in one of the units on the dealer lot during business hours for a couple of days and see what you like and dislike. We didn't care that people thought we were nuts.
Brand? Do some searches, depends on price range. Don't get suckered in by granite or things that just put lipstick on a pig. Quality runs deeper than stuff like that.
Problems? No not really. Stuff will break, you will get it fixed or replaced. Just like in a house or anywhere you live. Generally if you buy quality the issues should be low.
Cash is always better. Really depends on your cash flow and what you have going on if you can do it the other way. I'm guessing most people here can balance their check books and I'm sure you can as well. Don't go into this stretched to the nickle though.
Warranties? We avoid them. Generally junk that insurance companies make money on. It really is an insurance product, one they pay huge commissions on to dealers to sell. You do the math there.
Private property is an issue in most residential area IF YOU LIVE IN IT. We have 2 permanent sits we alternate between but spend most of the time in SE FL. Nice to have a place to park and stay if we want when ever we want but we also went 3 years without any place and it works just fine.
Replacement parts? Don't stress to much about that unless you do some camping away from civilization.
Repairs are generally pretty easy, mobile guys are generally hit or miss. If you are handy most things are pretty easy. If you aren't most things are pretty easy to learn if you take a few minutes and look at some Youtube videos. When all else fails get an appointment at a dealer and they generally have sites for you to stay at or at a minimum will hook you up to power while they fix your stuff. We've found most dealers very accommodating to full timers if you are nice and show some appreciation.
Buy your Fifth wheel first, then your truck. Over buy on the truck or you'll regret it later.
Noting wrong with used RVs. Noting wrong with new ones except you lose $$$ on them.
Good luck to you. Its just like living in a house, just smaller. We love it.