I build scale models and use airbrushes all the time for painting, primarily I use Iwata and Aztek airbrushes for all my work.
I would recommend maybe something from Iwata's neo range for starters. They are a little better quality than some of the cheaper chinese manufactured airbrushes out there but nowhere near as good as the Japanese manufactured offerings from the more premium range.
Harder & Steenbeck also have some great offerings in their Infinity range as well, a better budget airbrush than Iwata's neo offerings.
I suppose it's what you want to do with it will determine the best route for you to take. I do an awful lot of fine detail work so my Iwata HP-B with a 0.3mm needle suits me 99% of the time.