I went with the cheapest 25l unit from Machine Mart and have been more than happy with it (just by a big stereo or ear defenders!). Paid about 90 quid for it IIRC, and as it's oil-less it's perfect for airbrushing. Just remember that it will probably be 1/4BSP fitment, and most airbrush hoses or 1/8 BSP, quick search on EBay turned up some handy-dandy converters.
It's more than enough to run an airbrush, if you want to go HVLP for spraying liquid mask ot will be fine up to about a .8 touch-up gun. The main thing to look out for when running guns rather than brushes is the CFM (i.e. air volume) the the compressor can drive.
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