i've painted buses at least a half dozen times.
a cheap gravity feed hvlp gun from harbor freight or northern tool is the way to go in my opinion. Easy to control the spray, nice even pattern and about $50 bucks i think. There are about 3 moving parts inside the gun, and it can be totally disassembled without any tools for easy cleaning. I prefer the gravity feed because it has the paint reservoir ABOVE the gun. This makes for much more consistent spray than having the reservoir below. I also run mine at pretty high pressure....at least 120 psi at the tank. I find it does a much nicer job at higher pressure even though hvlp = high volume low pressure.
you want one that looks similiar to this:
this one is $59 from harbor freight. You want one that holds about a quart of paint. You're arm shouldn't get that tired, it should take way less than an hour to paint a bus. All the time and effort is invested in masking. Remember, 2 lighter coats rather than one heavy one. The 2nd coat is also nice because you can hit all the little spots you missed the first time, like inside the fenders, or on top the rain gutter etc.