Ferni, here's a couple of threads you can check out... There's a lot of good info here on airbrushes.
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=29222
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=28713
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=26148
I agree with Marburg, double action is better, but if single action is all you can get, it will work, it's just won't be as good at detail work.
There are two types of paint feeds too, siphon feed, where the paint is sucked up into the airbrush from a bottle. And gravity feed, where there is a metal cup on top of the airbrush that lets the paint flow down into the brush. both will do the job, the gravity feed is better at fine detail because it can spray at lower air pressures, but the siphon feed still does a great job all the same.
It all boils down to how much you want to initially invest in an airbursh. Buying a cheap one is not a good idea, there are lots of inexpensive (less than 60 dollars) airbrushes that will do everything you need and are of good quality.
You'll also need an airsource: either pressurized air cans, or a compressor. I vote for the compressor, at $69 dollars or so for the cheapest compressor, more for the quieter modeling types, it's going to save you a lot of money in the mean time. The air cans range from 8 to 10 dollars each... so after your first couple of models the air compressor would begin saving you money!
Read a little in those threads then come back and you can get more in depth details!
Good luck to you!