here's a couple of good links for you to read up on airbrushes etc... MusicCity here put them together to help out
http://www.craigcentral.com/models/ab.asp
the list of other topis he created are in this message here:
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=37953
personally I would go for a double action internal mix.. once you get comfortable with it you will be able to do more with it... the Badger 155 is an awesome brush for 59 dollars on www.dixieart.com