I have been doing flames and grafix on tanks for 5 years, I started out with a Vega 2000, but after my last one broke (used to buy them by the half dozen or dozen from Bear Air and got a break), I tried out the Iwata HP-series syphon-feed airbrush and it's GREAT! A little heavier than the Vega, but a MUCH STURDIER brush! The HP-C is a gravity feed, but only has like an 1/8 ounce cup...it's terrific for detail work, but if you're really looking for something that'll paint the pubes on a gnat, you should look into the Micron line...I use my syphon-feeds for regular work (like drop-shadowing flames, and for doing the fuzzy-outline for ghost flames), then go to the Micron or HP-C for the really tight, close-in detail work. The Micron is in the $260 range, but if you're looking to do murals or extremely tight details, it's worth it. If you're just looking to outline ghost flames, or drop-shadow, the HP-C may have a bit-too-small-of-a-cup to hold enough paint to do everything you're looking to do without refilling, but it's essentially the same brush as the HP-syphon feed brush, except that you can run it at lower pressure, and get better detail.
As far as the Aztek...stay away from it...they are really designed for T-shirt artists, and although they say they hold up to urethanes and laquers, they're mostly plastic, which doesn't handle thinner very well....
The 'hopper