I've used airbrush on some figures, it's not as hard to do as you might think. I got my airbrush on ebay for like $50 bucks and it came with a compressor and hose and everything you need. The difficulty with it is keeping it clean and mixing the paint properly, but once you get the hang of it, its easy.
I've used an airbrush on my RA Firefly, RA Iron Grenadiers, RA OC 101 Street Gang, basically, any figure that has faded camo.
I don't prep the plastic. I use Testors acrylic modeling paint so I never needed to prep the plastic.
As far as bushes go, I have a wide variety of different sized brushes. Detail brushes are a req. for face painting. You can get it all on ebay nowadays.
Feel free to ask any other questions. Happy to help out.
Not that I want you to give away your secrets or anything, but do you paint with a brush mostly or use an air brush or what?
Do you prepare the plastic beforehand? Light sanding or plastic primer?
What sort of paint do you use?