Air Brush. - Model Railroader Magazine

I have a Badger 150 and love it.  If you just need an airbrush for one project, I'd get the cheap one from Harbor Freight; if you see yourself using it for years for a number of different projects, I'd spend more money.  I'm partial to Badger (but they are all (for the most part) equal in quality,what they can spray, how well they spray,etc); check the availability of accessories, spare parts and support for whatever brand you choose.

Go for the Dual Action, it's just easier to work with (just pull all the way back and blast out any clogs or pull the needle back and, with no air, clean the dry paint from the nozzle with a Q-Tip, without risk of damaging the needle tip).  Also, see how easy it is to take apart to clean; a BIG part of airbrushing is cleaning everything really well (any serious retailer will show you how to break it down and clean it).  Hint: use lacquer thinner to clean it; it just about dissolves dry acrylic paint in seconds.

Lots of videos (not just for MRR) online for airbrush lessons/tutorials...very helpful for beginners (like us).  Do the exercises, spray thin horiz lines to fill several sheets, then make vertical ones; next, try a wavy lane, 'dagger' strokes, dots, fades, etc  You will use all those techniques during weathering.  Don't forget the dedicated airbrush artist forums; folks there own several (dozens even!) airbrushes and have opinions for each, depending on the job at hand.  Just don't get too distracted with airbrushing (those guys do amazing stuff!) and remember, you have a railroad to build/paint. :)

If you can afford it, get a cheap one, play with it, and upgrade later if you want to; or save buying a cheap one and go for any 'pro' Internal Mix, Dual Action airbrush (get a bottom feed so you can use a bottle, the built in paint cups are pretty small if you plan on spraying a lot of color (my opinion).  You can thin just about any paint, but I picked up a 'ready-to-spray' paint sample pack (5 colors) and practiced with that on paper (coloring book with good quality paper) to get used to the dual-action.