While many will tell you to go out and buy something basic, there's a few things to consider first.
How often do you build models? Are you a casual builder, or is this something you'll pursue with vigor? What do you model? Armor, aircraft, ships, cars? What is your budget? Do you want something to mess around with, or are you looking to make an investment into a tool that will bring you the results you want later on, once your skills have improved?
If you're looking to get a good quality airbrush that you will be happy with 2-3 years down the road, then don't waste your money on single action or use propellant cans.
You can buy a decent compressor from your local home-improvement store for under $200, and a Badger 150 or Iwata HP-BCS will cost less than $100. Then you've got a rig that you can use for the next 20 years.
Jeff