I agree with the sentiment to get a name brand rather than the 'blue box' Chinese brushes for a beginner. Getting the hang of an airbrush can be difficult enough in itself, without any additional issues associated with poor quality.
As to type, absolutely dual action gravity feed, I have just been though an exercise getting Jo Sonya craft paints to work through suction, side feed and gravity brushes, and the gravity feed was by far the easiest to get a good result.
Brand-wise, I have nothing but praise for my Harder & Steinbeck Infinity, which is admittedly a top of the range brush but their evolution range may fit your specs. Apart from the performance, the design of the H&S brushes makes them a little easier to break down and clean than the 'classic' Iwata and their ebay clones. If you use your airbrush a lot, you will be cleaning it a lot, and anything that makes that task easier is a very good thing! If you require a local stockist that may be an issue, but I'm in Australia and got mine from everythingairbrush.com in the UK and have had no issues.
I've had no experience with Co2 cannisters, but can say that I use a AU$87 basic eBay 1/6 hp compressor with air tank and had no real problems.
For water based paints, one of those fold up portable booths is a good idea. A dusk mask may protect you from particulates, but it does nothing for the fine layer of dust that can accumulate everywhere else. I have a similar non-foldup booth from Sparmax and there is a definite difference to the air quality when I use it.
Cheers!
Joe Thomlinson