Floxy, there can be an alternative : some art shops give airbrush lessons; depending on the place you live, there may be one in the surroundings. Although it will be more art oriented, basics are good for everyone and you'll have opportunity to try before you buy. Apart of this, go see the sites the other chaps quoted as they are full of information. If all this fails, you can try by yourself, as an airbrush is much less dangerous than a gun (don't aim at yourself, though). When I get a new airbrush, I try first with... air alone, to get the feel of this new engine, then I put plain water, then I try with ink. After that, there is more possibilities that I can speak of here, but the topic is : feel it in your hand, see how it behaves, how low you can set the pressure, and how high, what will be the thickness of the medium you will spray,etc... in one word : practice.
Boss, when you spoke of only four brands of airbrushes, I think you forgot Aerograph/De Vilbiss, Harder & Steenbeck, Hansa, Richpen, Holbein, Efbe, and some more. Okay, there is a number of European brands in this list but they are worth trying.