In line with your budget - consider this...
http://www.graphicsdirect.co.uk/prod...ction-airbrush
It's what I use, and it's a massive improvement on the AB brush I had previously from everythingairbrush.co.uk
Not tried an Iwata myself, so that will I'm sure be better still, but I get on absolutely fine with this one and the build quality is good.
HTH