I hope I don't step all over your thread too badly, but I used to build model tanks when I was a teenager and young adult, and airbrushing is a whole 'nuther field.
Kind of like some people spend so much time reloading that they never get to shoot, airbrushing is a skill all its own.
I found out early on that airbrushing is a skill that can take as much time, effort, and practice as any other hobby. I compare it to learning a musical instrument. You just don't pick up the thing for the first time and expect it to do what you want.
Having said that (and I hope I didn't just spew a bunch of stuff you weren't asking about), a rule of thumb is that the more expensive and versatile equipment really is better.
You can buy anything from a cheap unit that won't do much, all the way up to a professional grade airbrush that will literally draw anything you want.
You don't say anything about using it for camo patterns, but on a firearm I see that as the most useful job for an airbrush.
Oh, and the worst thing about airbrushes: you have to clean 'em every time you change colors. You have to thin the paint or whatever just right. If it's too thin it won't cover the object well; if it's too thick it will clog the airbrush. The thinness of paint required can also change from one type and one brand of paint versus another (although I make it sound worse than it is).
Just my humble two cents.