Secondly £100 will buy you a very good airbrush. But you don't say what experience you have with airbrushing. I personally would start off with something cheap (but still a 2-stage) just until you've got some experience with them. There are a few different type of feed systems. Gravity, side and bottom. I prefer top feeds but it's user preference.
I bought a really cheap 2-stage from Machine Mart first off then, a better one from Everything Airbrush. Finally i bought an Iwata CR Revolution from John in Scale Models shop
I have very little experience so I will look into the 2 stage you are talking about. I am a complete rookie so do you have any recommendations of a 2 stage airbrush? (Probably asking a really stupid question, sorry)
You can buy cans of compressed airbrush gas but it is really expensive in the long run. Maybe someone else on the site knows of an airbrush/compressor combo you can get for under £100