A friend of mine recommends Iwata airbrush kits. See link below. I am sure you can find it cheaper on another website, but I wanted to give you a link so that you know what to look for.
The Paasche VLSTPRO is what most professionals use for t-shirts. It's a workhorse and you'll need it, believe me. The Createx paint you'll be using is a job in itself (lol)...you'll find out. But Airbrushing T-Shirts is so much fun You should also ch...
How much paint really depends on how big the resevoir is, and how big you want to paint. What's more important is how thick the paint is. Thinned watercolour paint works great - it never clogs the tip. It should be the consistency of cream. Use water...