First, you need to know I`m far from being an expert with an airbrush.
I would suggest you get as good a brush as possible. A double action will do anything you train yourself to do. With proper care and maintance, it will last many years. Fine lines are easier to achieve than with a single action.
Look at this site for more information.
http://cidwebs.com/armorinscale/airbrush.htm
Do a google search 'airbrush basics' for more info.
http://www.google.com/