at the very least, I suggest getting an Iwata Revolution, and some form of an air compressor, I have the SilverJet, it works fine. They're definately not cheap. I'm new to airbrushing, and I knew that right away. I knew that I was going to have lots of things to paint, and I wanted something I could hand down. Look to spend at least 250 for a good one. I highly recomend a double action airbrush too, if you can. And don't mess around with the needle!
I need to get replacement parts for mine. (HP-SBP something... not cheap!) If you don't want to do highly detailed bodies, I really recomend you stick with masking, and rattle cans with a narrow spray pattern. I've had the Testors airbrush... ack. It's a pain, you've got to be willing to handle the maintennance of an airbrush too. You have to constantly clean it while you're working.