Depends on what you're doing, and where you're doing it.
For smaller jobs, an airbrush will do. Look for a dual action type. A small diaphragm compressor and a small air tank to help balance the airflow are good enough and are relatively quiet. (just tossing up some pics, not suggesting a specific shop)
If you're looking for an air touch up gun, you could get away with a small pancake air compressor to run it. Not as quiet but won't drive the neighbors insane.
For bigger jobs, go with a full HVLP setup. It gets pricey, but it's worth it to spend the money for quality stuff. You will need a rather sizeable compressor to run that with. We've got a big 80 gallon one at the air museum. You could probably go with one half that size, but still a rather large, expensive and really loud compressor.
Most likely would be the touch up gun. Get the gravity feed type (top canister), not the vacuum feed (bottom canister). Benefit to going with your own spray guns is also it opens up a whole world of paint types. Candies, multi-stage, clear coats, irridescent pigments and other things that don't come in a rattle can.