I'm no expert, but what you pay for is what you get.
you're looking for an airbrush that can vary the thickness in which it paints. sometimes you want to spray thin, other times you'll want to spray thick.
cleaning is another huge factor, it should be easily taken apart and cleaned.

I'm in the process of contemplating an air brush myself.
seems like about 75.00 should get me a great all around air brush that I could probably use for a lot and for a long time.

I'm not a fan of super cheapies, they're usually not worth the 20.00 spent on em.