My compressor is a SimAir AC100 and it's been excellent but it was a bit of pot luck rather than me actually choosing that brand since the eBay description didn't actually include the brand when I bought it! I suggested eBay as I noticed you have London as your location so the way a lot of our American friends bring the price down (coupons) is not an option on this side of the Pond!
You want something that runs between 20-50psi with the optimum being 40-50psi. Lower than 20psi and you'll get splattery ink droplets and higher than 50psi you risk damaging the Air Grip.
Something that's "on demand" is good since that means the motor will only kick in when the pressure starts to drop below the threshold rather than just running all the time.
You do need the Air Adaptor if you're going to use a compressor (or even the larger air cans, the D60 can is the only one you can connect directly to the Air Grip).
There's a good run down of the various system options
here that might help if you're weighing up pros and cons of cans vs compressors. The only thing I'd disagree with is the statement that only Copic Original and Sketch are suitable for airbrushing. That's the official line but you can use Ciao if you add a little piece of foam tape in the Air Grip to lift the barrel. Most of my Copics are Ciao and I use them to airbrush with no problem.
HTH!
Copic airbrush system
