If you are just doing basic blanket coverage and aren't worried about fine lines etc. the cheapest airbrush you can buy is just fine!
I use a siphon feed single action airbrush I bought around 15 years ago for £7.50 for applying spirit stains. I use that because I have a different bottle for each colour and I just switch bottles to change colours. You can buy the kits with a few siphon feed guns and a few bottles for not a lot of money.
If you want to use acrylic paints, the double action gravity feed types are probably better suited because you can just put a few drops of paint in the cup and spray. It's also easy to mix colours in the cup if needed. Choice of paints might dictate the type of gun. For the thinner paints such as Com Art and Golden etc. you can use a 0.2 - 0.3mm nozzle but some of the others like Createx need a 0.5mm nozzle or bigger.
Don't be fooled into thinking you need to buy an expensive airbrush unless you are doing very fine detail and even then, you may not need anything too fine if you are using masking techniques etc. The website below sells BD airbrushes of both gravity and siphon feed types. I have a BD and an Iwata and for the techniques I use with acrylics, either would do. The only significant difference between the ones I have is the nozzle diameter.
http://www.airbrush-paint.co.uk/contents/en-uk/d27.htmlIf you want to come to my workshop and try out the different guns, paints etc. let me know and we can arrange something.