I bought 4 gallons of paint, and I have about 1 1/2 left over (hmmm....that ceiling might look better in white than in green.....). I put two coats on with a roller, and I thinned the paint the recommended amount (8 ounces mineral spirits per gallon IIRC). I didn't use primer, but sanded down to clean paint with 100 grit and a 1/3 sheet orbital sander. I didn't have any rust to speak of anywhere on the bus, so I didn't have to deal with that issue.
If you use a roller, buy good roller covers. The cheap ones leave little pieces of lint that get pulled out of the roller in the paint. I don't know that the good ones won't, but I talked to a painter who said they didn't.
I rolled paint on a section and let it dry to the tacky stage while I rolled another section (about 3'x3' or so), then I went back with a damp, but depleted roller and re-rolled the tacky section very firmly. That eliminated air bubbles and "roping" (the streaks you get from the edge of the roller when you press too hard on it). Where the lint didn't come out of the roller and get stuck in the paint, it is hard to tell it from a sprayed on coat.
Disposability is your friend. Buy enough roller covers and some disposable gloves.