Hi GreyBeard,
Basically if you just want to slap a coat of finish on an item any airbrush will do the job. If you want to be able to paint or do finer work then you need to go upmarket. Most of the ones sold for painting fingernails/tanning etc are quite good.
There are dozens on ebay, a full kit (compressor,brush,regulator,hose etc) in the mid range will set you back about $75-150
I have 2 airbrushes,one mid range and one high spec, with a dedicated compressor for normal to fine work. I also use a touch-up gun, which is about 1/3 size of of a normal spray gun that runs off the workshop compressor for larger work.
Water based paints/finishes (Acrylics) are much easier to use and clean up.