I think Don is right on all points made, I would add it's likely best to keep the investment on the minimal side to start, the chances of your selecting the perfect airbrush first time around are quite low. Things to consider will be the model building you will do, (aircraft, cars, ships, armor, figures?) Type of paint, (acrylic, enamel, lacquer, natural metal finish or a mix?) Complex multi color camo, or other? Single action, or double action? And just how fine will the detailing be?
I have many airbrushes, several of the brands, have centered on Badger for years now. Reasonable cost, excellent quality and reliability, simple to use and maintain, they provide very good spray results. Of my eight Badgers, my most commonly used are the 200, 200-G and 100-G, they are capable of doing the greatest majority of my painting.
Bargains can be found at the discount dealers, I have made very good purchases from Chicago Airbrush, TCP, Webair and Dixie Art, quality, factory authorized dealers. I buy brand new whenever possible, but have made good used purchases as well.
Compressors: It doesn't have to be an expensive one, but do keep in mind that noise can make for an unhappy housemate and neighbors. Shop around, fairly quiet ones can be found for less than $100. Mine is a California Air Tools 1610A, very powerful, large volume tank, super quiet, wife can't hear it in next room. US $180. Be mindful of health preservation, as much as possible use the rig in the best ventilation area you can arrange.
Last, but most important: Go to "Don Wheeler's Airbrush" website, spend some time going through it, you'll be doing yourself a huge favor by checking it out thoroughly. Pretty much all there is to know about airbrushes, types, uses and maintenance is found there, compressors also. You're always welcome to check back with other questions, good luck with your search.
Patrick