I used a paint sold by Tractor Supply. It is a Val-Spar product called BPS Tractor and Implement Enamel. I sanded down to clean old paint, wiped with acetone as I went, and then painted it with a roller. People are amazed that I used a roller because it actually came out very well. I think it's best to use the shortest nap roller possible. Get good quality roller covers because they are less likely to shed fluff into the paint.
I used a 1/3 sheet Black&Decker orbital sander. It went quickly. I don't recall what grade sandpaper I used. Definately use a good sander, and don't be conservative with the sandpaper -- it's a lot cheaper than your time! When it stops cutting, don't push harder and try to get the very last out of it. It is a consumable supply. Save the pieces for hand sanding other projects, but do yourself the favor of changing paper often.
WEAR A MASK WHEN YOU SAND. No telling what's in the paint of a 25 year old bus....
IIRC, I did thin the mixture by the maximum recommended proportion (like 8 oz of thinner per gallon or something.)
I painted an area about 3'x3' (it's what I can reach), then moved on and painted two more areas the same size. THEN, I went back over the first area with the depleted roller from the third area -- that smooths out the few bubbles that might be left. It's important to let the first area get a bit tacky first, thus you are always working 2 areas ahead of the one you are going to go back over -- gives it time to dry a bit. It's important that you use a depleted roller for this, I think, and that you work the area pretty firmly with it when you go back over an area. The little bit of paint on the roller keeps it from sticking to the tacky paint, and working it firmly helps to smooth out any little air bubbles left by the roller.
I found that if I didn't re-roll an area, I would invariably end up with pin-prick bubbles here and there.
Brush or blow off the bus VERY WELL if you used any steel wool. The tiny fragments of it like to hide in nooks and crannies and then get into the paint, where they rust and leave little tiny rust streaks in the paint.
I only put one coat on. That was 5 years ago, almost. The only places where it is coming up are small dings where the paint got nicked by stones from the lawnmower, and a small spot on a sharp corner on one of the roof ribs. When I wash it it still looks nice and white. Not quite as shiny as it once was, but not a weathered, flat white.
For the 2 small spots of rust on the body, I used Rustoleum Primer for Rusty Metal. Rustoleum says that you should not use the rusty metal primer on good metal, so I didn't. But I only primered those two rusty spots. The rest of the bus I just took down to fresh(er) paint.
Here's a pictorial of how I painted mine.
[added]As you can see in the picture, I couldn't have used a spray gun in the location I was working.