If you're new to using an airbrush, I would avoid the double actions. I would recommend either the Paasche H, Badger 350, Badger 200 siphon feed or Badger 200 gravity feed.
The Paasche H and Badger 350 are both siphon feed, single action and external mix. Many modelers started with these airbrushes and still use them. They will cost about $50 to $60.
The Badger 200 is a single action, internal mix. The spray tends to be more fine than the external mix airbrushes. The Badger 200 can be a little more difficult to clean and maintain than the Paasche H and Badger 350. They can run from $60 to $100.
Remember, an airbrush is a tool. A tool is only as good as the user. Buying a $150 airbrush doesn't necessarily mean you'll get a better paint job. I say, learn the basics (paint to thinner ratios, air pressure, painting technique, etc) before spending a lot of money on an airbrush. Would it make sense, when learning how to drive, to start in a Ferrari?
-Jesse