you don't really need an airbrush to get a nice finish. It helps with the more complicated paint schemes, but for monochromatic finishes, you can use a rattle can (like dupli-color or krylon). And I've seen guys with brush painting skills that put some people's airbrush skills, to shame.
The most important tool you need, is practice. Find a garage sale or flea market or even check ebay, for cheap model kits. I'm talking $1 - $5 cheap. Practice assembly techniques. Learn how to properly apply glue. Learn when to use thin cement vs. crazy glue. Learn to fill and sand seams. Then learn how to brush paint so that it looks like it's been airbrushed.
Dropping $100 on a decent airbrush isn't going to make anyone, any better of a model builder.
So, for the basics, I recommend a few cheap models (subject doesn't matter), a decent hobby knife, some hobby-grade sanding sticks, thin cement and some cyanoacrylate (crazy glue) and some decent paintbrushes in different sizes, from fan brushes all the way down to 20/0 liners. For brushable paints, I recommend Vallejo and Testors Model Master acrylics. They clean up with water and brush on smooth.
All the tools (sanding sticks, glue, brushes, etc.) can be found online at places like micromark.com, towerhobbies.com and megahobby.com.
Be sure to ask plenty of questions and most people here will be more than happy to help.
And don't forget to check out dedicated modeling sites like finescale.com, largescaleplanes.com (lsp) and AircraftResourceCenter.com (ARCforums.com).
-Fred