sand the outside (no need to take all the paint off, just rough up the surface so the new paint will stick), with either sandpaper or steel wool..... wash the bus, let it dry, use blue masking tape and newspaper to cover surfaces you don't want painted, and even remove reflectors and lights that you can..... if you have rust already you need to sand all of the rust off..... if you have holes in your bus due to rust, sand and then use bondo, for either of these, use rustoleum primer (I prefer spray cans because if you do it right the texture is much smoother), if you don't have rust, painting it is all you need to do to prevent the rust..... you can prime it if there is a dark ccolor on it now, and can afford it, or you can just spray paint it-- but if you have never spray painted before, talk with some local graffiti artists, as there is quite a technique to spray painting.... and if you do it wrong, the paint drips and looks awful...... you can also use regular exterior latex or acrilyc paint with a brush or roller... we have done that on a couple of our busses... it is cheaper and easier, but looks less professional, and gives it a texture (which can be nice), but it absorbs the exhaust more and discolors in the back quicker......... if you want it to look nice, this will be a long project, as you need to put at least two or three coats of spray on, and then there is trim, etc..... you need to allow a lot of time for the spray to dry, and can't spray in cold or wind......... on the inside of the bus you can use regular interior acrilyc/latex paint, after sanding....... the inside of our bus has had the same paint job for 5 years and it is still going strong...... plan on being off the bus for a day or so for it to dry though, or at least, not touch it...... if you have any more questions, or if anything I said isn't clear, feel free to ask me anything.... I've painted a lot of different busses in a lot of different ways........

Wash BEFOR you sand, and use scuff pads, steel wool has oil on it to keep it from rusting. Get off the loose rust and use Rustolem primer for RUSTY metal. It's differant from other types.
Ext. simi-gloss house paint. After 3 days of dry weather, a coat of liquid wax, type doesn't matter. but it should be liquid. this seals the paint and helps keep it cleaner.
AS far as the eng. , If'n it ain't broke , don't fix it !