Von, aye, the necrons are probably the only army I could paint in a reasonable amount of time
County, to thin normal GW/acrylic paints you can use either water, or acrylic thinner.
Water if cheap, but you lose some of the adhesion with it, and Vallejo do big bottles (500ml) for under a tenner of proper acrylic thinner which doesn't affect the paint's adhesive qualities as much (although IIRC you can't use it with some metallics).
You can get cheap twin action brushes on ebay (often a pair of them with a small compressor for about £70-100), or you can pay more and get a named brand that you'll be able to get spares for (I use a Harder & Steinbeck Evolution).
The main thing is to get a top feeder twin action brush, as bottom feeder brushed tend to require higher pressures to work (they need to effectively syphon the paint up from the bottle), and can't get into close spaces (due to the large bottle under it), and twin action so you can start the air, then start the paint flowing, which gives you massively more control.
Ideally you also want to get a face mask (about £20), and you will need to allow for things like airbrush cleaner (about a tenner will get you enough to last ages).
We went over the top a bit with ours, going for a good brush at ~£140, a good smallish compressor at ~£200, and then a proper extractor at ~£250 (mainly because we were fed up of the haze spraying cans of undercoat created in the garage), but you can get set up for about £100 all in if you shop around, and don't worry about getting name brand stuff.