A good quality airbrush, single action, will cost you about $70 or more. HF sells their brand of an Aztec, but how good it is is debatable. I personally would recommend a Passche H model. Spend a little extra and get a #1 tip for it as well. They generally come with a #3, which can spray a larger pattern, but I've found that the #1 sprays a finer layer of paint. I have done over 95% of my custom painting with this brush, and the only parts I've ever had to buy for it are new spray tips. Extremely dependable and rugged.
Badger also sells a good quality single action brush in the 150. Its advantage is it has a needle that can be set for whatever size pattern you want, so no changing tips. It was my first brush, but lasted only 3 years, and couldn't be repaired. That's when I got the Passche. I haven't looked back.
The $20 HF brush that folks have been talking about on here apparently is a hit or miss brush for some folks. I do a lot of painting, I have been a custom painter since I was 19, I'm in my late 50's now. I've even bought a HF airbrush to try it out, and I have had absolutely
NONE of the problems some others seem to have had. The pull and push, its a double action, is extremely smooth and the spray pattern can go from extremely thin to almost 1.5" in diameter. I don't know if its my luck with AB's or what. Just remember what Alan, (Espeefan), says about it,
YMMV, Your Mileage May Vary!
But I would like to confirm what MRLDave told you. A single action would be the best way to go for learning. Others will tell you to go ahead and make the investment into a double action brush, but I do disagree. What if you really can't get comfortable with a good double action brush. While you can set up a DA brush to function as a SA brush, it would be a waste of money to do so. I do everything with the SA Passche except what I call extreme weathering, extremely small rust streaks on loco engine doors, bird poop on roofs of structures, chalked "graffiti" on boxcars from hoboes, etc. For that I use a Badger XF100 extra fine DA and a DA gravity feed with the smallest tip installed. While I do use the Passche for most of my weathering, I use these extra fine brushes because they use such a small amount of paint, and can give me the most subtlest weathering.
I would also tell you to get an airbrush made from metal, and not from plastic. Metal will wear a lot slower than plastic will, and a brush made out of plastic will need parts long before a metal one does.