There are tons of different airbrushes that you can buy, so I won't go too into detail with brands. I personally, however, recommend Paasche or Iwaata airbrushes, if you have the money for one. You can get a pretty decent airbrush kit on eBay these days for under 100 USD.
When choosing an airbrush, you will definitely want to buy a dual action airbrush. With a single action airbrush, you simply push down the trigger for air and paint flow. To adjust the amount of paint flow, you have to stop airbrushing and change it. With a dual action air brush, you push down for airflow, and back for paint flow. This allows total control of how much paint you want to release, thus enabling you to create very wide or very thin lines, very easily.
[[1]] The Paasche VLS Dual Action Airbrush Set
The only other main thing to worry about when buying a beginner's airbrush is siphon or gravity feed. Gravity feed airbrushes don't need to operate at as high of pressures as siphon feed airbrushes, since the paint flows with the help of gravity. With the lower pressure, you can attain higher detail. So, gravity feed brushes usually go for a bit more than siphon feed brushes. However, with my siphon feed Paasche VLS, I can easily get a 1/32" line. So, it's up to you, really. Read into them a bit, and make a good choice. Really though, as long as you get a dual action airbrush from a respectable manufacturer, you should be just fine.
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