With 20 quid, you wont get much, only a single action, external mix jobbie, like mine, which IS NOT RECOMMENDED!!
Try this one, (the second on the page) which is very good value, and gravity fed, so you waste much less paint, which, although it is £50, will be very good for what you would use it for.
http://www.everythingairbrush.com/acatalog...ial_Offers.html
If you already have a compressor (you will need one of these, rather than a cannister of proplellant, as these are expensive, and are used up quickly) you can buy it seperately for £35 quid.
Try any one of these:
http://www.every