Airbrush newbie here
I'm a research guru, and I did a lot of homework before buying my airbrush. The fine folks here at The Gauge were a great help as well.
LongIslandTom and
doctorwayne have been especially helpful in this decision.
For model railroaders, you're probably looking at two main brands for the airbrush: Badger or Paasche. They're both good quality airbrushes at reasonable prices. Iwata (I think that's the name) has some high-end equipment, but it seems like overkill to me.
The biggest decision as far as type of airbrush is either single or double action. With a single action, you press the button down and both paint and air come out at the same time. With double action, you press the button down for air then forward for paint. Most here agree that double action gives you more control, but can be harder to use.
I've used the compressed air cans, and they're junk. I needed to paint, and I couldn't yet afford a mini-compressor...so I did what I had to. I bought a compressor, and just today fitted it with a regulator and moisture filter. If you're interested, you can read the thread on my paint woes
here.
For the most part, I use Floquil or Polly Scale paints. Both are made by Testors, who specializes in model paints. Those particular brands are for model railroading, and the color names are very easy to use (eg. Dirt, Grimy Black, Oily Black, Rust, Freight Car Red). You can use other paints, but realize that you're going to be spraying them at low pressure, so they will most likely have to be thinned down.