We purchased a bus this summer and decided to go with a short bus. If the vehicle is designed to carry more than 16 people, then you technically have to have a CDL with passenger endorsement. Yes, you can get around this by making it an RV, but as someone pointed out, it has to have several things(toilet, bed, sink, etc) to get it classed as an RV. If you keep it a regular bus(not an RV), the insurance will run about $100/month. If it's an RV, it will cost you around $300/year. Both of these are assuming liability only. You can definitely count on more expenses than planned. We had to buy new tires, new brakes, distributor cap, and have some other maintenance done to make it road worthy. We initially had 5 "investors", and ended up with 2...me and my brother. I would recommend getting several people. Like someone else said, make sure they are committed. Have everyone throw in $200 right away into a maintenance fun.
With all this said, our bus is a blast!!! We modified the seats for perimeter seating, installed new stereo and TV, got some actual field turf for flooring and painted a football field on it, and installed a kegerator in the back with a tapper out the side of the bus. If you want to see it, we park in lot D2. It's white with ISU decals and license plate says BUS4CY.