which is the right airbrush

Hi Doc,  sorry for a tardy reply - but haven't been here in awhile.  Speaking from my own recent experience I bought two airbrushes at Nationals and I love them Both.  They were both Badger's (they had some great show specials).  The Patriot is a little more budget friendly and has given me pencil lines when rarely needed and does all my camoflauge finishing spectaculary without any complications.  The Renegade Velocity gives even finer detail, but doesn't get as wide on the braod end of spray patterns.  For most finescale modeling needs the Patriot is ideal for fine and general purpose apps.  The Renegade is for museum quality or contest entry detail work. 

I am using the Patriot about 80-90% of the time, and the Velocity for the 10% of real intricate work I want to do.  Since getting these my bottom feed guns have been pretty much laid to rest.

With regards to an airbrush, you should go for a gravity fed twin action.

Gravity fed - better for detail as less paint wasted, easier clean up, lower pressure needed than syphon (better for detail).

Twin action - because you will regret not getting one once you have become accustomed to a single.

I used a Testors Mini-blue for many years. To be honest, it "did the job" in the sense that it was more than good enough to apply basecoats, and some effects, such as general dusting, etc. Most of this is somewhat irrelevant to airplane modeling, except the  basecoating. It can be used for just general work, and you can do fine camo with it as well, provided you use masks. However, it really will not compare to a good two-piston (as I've recently discovered) design with a regulator and moisture trap.

I suggest you at least try it, it should be more than good enough to learn and practice a bit, before you make the plunge on a more expensive hobby compressor.