For an inexpensive trial . . . check out Harbor Freight. They have cheap but not junk airbrushes all the time, and occasionally put them on sale for even cheaper! They are simple single action bottle-feed units, but they'll allow you to learn the process and the feel of airbrushing.
They are fine for basic painting, especially workman-type stuff - chassis, engines, interiors, priming, the basics. They'll do fine for starter body painting practice. As you get a feel for the airbrushing process, you'll want to graduate to better quality ... but the Harbor Freight cheapies are great to keep around for the quick, basic stuff.
Just my 2-cents.