Airbrushes are purpose built. Some are really good at details, others are really good at priming and basecoating. Some are okay at both. Before you buy an airbrush, you need to ask the question "How do I intend to use it?" and then find a brush that matches that need. If you're only looking to cover large areas (like when priming, basecoating, and varnishing) and indend to pick out details by hand, you can probably do okay for cheap. If you want to learn airbrushing as an art, it's worth saving up a little longer so that you can afford a quality airbrush or two.
My workhorse brushes are an
Iwata Eclipse HP - CS and an
Iwata Eclipse HP - BCS. After burning out some cheaper compressors, I finally settled on a
two-cylinder model with a tank. With this setup I can paint a huge range of details with pretty much any paint than can be thinned (including craft and house paints). For more basic needs or occasional use, the link that Todosi posted would be perfect.