For Jigs, I prefer powder coating. Less initial outlay.
For air brushing, buy the best you can afford. But you probably should look into an airbrush basics class at a local arts center or vocational school. I thought I was pretty good with one until I took a 3 week, 4 hour every other saturday class. The class was 150 dollars, you got to use a dozen or so different air brushes and learn everything you ever need to know about air brushing.