Get a big bus. The smaller buses are more expensive believe it or not. According to my father in law who is a truck driver, you only need a CDL for driving commercially. There's a weight limit in most states on how much you can weigh with a regular drivers license, but there isn't a school bus in this country that's going to exceed that. A big bus is going to use a little more gas, but you can fill it up with more people to help pay for that gas!
I'm planning to buy a short bus next. I'm going to put a couple bunks, a tiny bathroom that has a toilet in the shower, a small kitchen area and a small couch. It's going to be set up for two people. Three max. I'll use that as my hunting rig or the bus I'll take to dirt bike races when my family doesn't go with me. For your purposes though, you really need a big bus.