Many many ways to go. A lot of people touched on a lot of different things. This is a general what I know list.
Things you just upkeep, redo I.E. not build already done
Old RV's normal stuff in the price range - Generally check the roof. they are notorious in any old one. which you pretty much have to tear apart the roof, sides, and otherwise to remove rotten wood, replace, recover and reseal. Also, the plumbing. Thats the short falls off hand.
Someone else's conversion of bus or otherwise - Could be nice, could be bad.
Truck bed camper - for one or two people, you can find these cheap. pop-up top versions are nice and cut wind resistance. though, pay attention, Canvas is easy to replace but can be costly. If you travel with others, it is nice to go to the back and make a sammich. You cant do it with these, but otherwise nice
Thing you may build
Ambulance - Normally solid and pretty good. Can take a bit to convert. may be hard to do some things from a space constraint aspect
Bus - I like it, normally, if they leak, its a window seal. which is easy. Huge storage potential underneath with them. You would have to add all of what you want. But you can find shorter versions with teh same engine for a sport like ride :).
Box truck or your go everywhere behemoth - room, same as the bus, just takes work.
For a cheap RV, I am scared of the wood rot. Its a real killer and can turn a cheap option to a ton of sweat and cash to fix properly. They seem to rot fast too. Take your time and weigh options. look at the recycle places for what prices are with bathroom stuff to see if it makes sense to try to do a build. For a marginal make it work, it wont take much, and you can add as you go as budget works.