Are the dirt bikes 2-strokes? If so, just drain the fuel and lay 'em on their sides in a bay (in a way so they don't bank around, of course). If not, or even if so, I like the idea of a van to put 'em in. A regular or extended (15 passenger) E250 or 350 van would hold a lot of stuff and not put much tongue weight on the hitch, like Uncle Ned has mentioned. This is what I'm going to do for my smallish Yamaha Virago. A drive shaft disconnect such as www.remcotowing.com/drive_shaft.php
would prevent transmission damage. It'd be a good vehicle for getting stuff for the bus if you don't already have a van or truck. Alternately, would they fit in the back of a 4x4 pickup? If it has 'N' on the transfer selector, you're all set! Putting the bikes somewhere other than in the bus itself would give you a LOT more options for bus selection. $10K should buy you a nice MC-8 or -9. They're only 96" wide, but unless you've got a lot more cash, want a transit bus, or are going to be living in it, those two are good models. Parts are readily available and inexpensive for both.