Welcome to the boards!
For the best results with an airbrush, you'll want a double action, internal mix, but before you commit yourself to an expensive airbrush, I'd suggest picking up a cheap one to make sure it's for you. Not everybody enjoys using an AB, so it's better to find out when you're only in the hole for a couple of bucks. I'd suggest getting something along the lines of an aztek ($50), or if can spend a few bucks more, perhaps an Iwata Revolution($70).
To help with some of the terms, here's a quick breakdown
Double Action - brush has a single trigger, pushing down on the trigger opens the air valve while while pulling the trigger back open the paint nozzle (by moving the needle back). This set up gives you best control over air and paint volume. These are internal mix brushes
Single Action - brush has one trigger, pushing down on the trigger open the airvalve. There is no control over the paint flow. The air moving across the paint feed (cup/bowl) creates a vaccuum and draws paint into the airstream. These are typically extrenal mix brushes.
Internal mix - the atomization of the paint in the air stream occurrs in the brush, before it ever gets to the nozzle. You have much more thorough atomization.
External mix - The paint is introduced into the airflow at the very end of the brush, just outside the nozzle. Atomization is not as fine as an internal mix.
Once you get a hang of how the double action works and you decide you really like it, I'd suggest getting one of the Iwata Eclipse series brushes($110). For fine line work, I suggest a gravity feed as they allow you to work with lower airpressure (psi). If you're going to be doing larger areas, consider a bottom feed. These hold larger volumes of paint, but require a higher psi (vaccuum draw) to work.
Hope it helps some,
Fred