I'm no airbrush whiz, but I've been very happy over the years with Paasche.
You can pick up a single action siphon airbrush like the Paasche H or HS for under $30 new.
After you get a little experience, and decide it's what you like doing, then move to a double action like the VL or VLS. (I have been using a VL for years and love it.)
The only real advantage you get with a gravity feed is you can do a *slightly* finer line and run a little less air pressure for the fluid you're spraying. Unless you're doing it for a living, save the extra $$$ for now and just get the feel of airbrushing.
As cheap as new good quality siphon feed units are, stay away from eBay and used units...often they haven't been maintained or cleaned properly, and it's more expensive and frustrating to rebuild a damaged gun as it is to buy a new one.
Two online merchants I can recommentd:
Dixie Art & AirbrushBear Air