There are in reality only a few makers for airbrushes, don't be fooled by the names many Airbrushes come from the same factory.
Example:
BB Rich makes airbrushes for Tamiya, Gunze Sangyo, Iwata, Olympus, etc.
And, yes, their parts are often interchangeable.
Personally, I would save up a bit of cash and buy one of the top-end airbrushes right-off.
Too many people happilly spend $150 on a Kit, but squirm when they have to fork out $80 or more for an Airbrush.
Get a cheap airbrush and for the most part it will perform on a lower level than a top-end airbrush.
Apart from that see if you can hold and try as many airbrushes prior to making up your mind which one to buy, a good airbrush alone won't deliver a good paintjob a LOT also depends on the person using it and how comfortable that person is with his airbrush.
Like many I have gone through a selection of airbrushes prior to setling on my current setup and don't plan from changing from it.
HTH.