This is a tad like saying I want to buy a car................. There is a huge selection of air brushes available and you should have clear what you want to do with an air brush. If you want a smooth base coat go for a single action ( I have a Badger 150 for that). For shading, camo etc. try a cheap double action air brush ( I have a Harder and Steenbeck Ultra with 0,2mm needle, cost 69 Euros and an Iwata Revolution with 0,5mm needle, 89 Euros). Another nice set I picked up for a very reasonable price was an ex-East German Gabbert Triplex, (wooden box set containing 3 heads with 3 different needle sizes. Very utilitarian but works like a charm).
Once you feel confident in using the brush and have more idea what effects you want to achieve you can splurge on a more expensive brush. I would also advise buying a good compressor to use with the brush. you will never get satisfying results with cans of air powering the brush, (I have a hefty garage compressor with huge storage tank and moisture trap and it only cost about 110 Euros).
Good hunting,
Julian