Ah HA! An art thing I can answer!
I've been messing with airbrushes for a couple of years now, and I _love_ airbrushing. It's weird, because I was never really an art person...
Currently I have two airbrushes. Here they are:
The top one is a Badger 350: It's a VERY basic airbrush. External mix, single-action, siphon fed. It cost under $50 for the airbrush. It is functional, but very limited.
The bottom one is a Badger Anthem 155. It's a good, solid, higher-mid-range airbrush. Double action, internal mix, still siphon fed. It cost about $80, and runs from $75-100 depending on what's packed in. This is what I'd recommend, with one reservation.
Airbrush term time:
single action -- you have one button that turns the air on and off. you have no control over the amount of paint as you go. It's good for base coats, but not good for detail stuff.
(the opposite is)
double action -- you push in the button to turn the air on, and then you can pull the button back to control the amount of paint. This gives you a lot of control-- with a good airbrush you can draw almost pencil-like lines.
Internal mix -- the airbrush is designed so that the paint is sucked into the middle of the airflow. this gives a nice round paint pattern
(the opposite is)
external mix -- the paint is sucked up into one side of the airflow. this gives a wider, (-shaped paint pattern.
Siphon fed -- paint feeds into the bottom of the airbrush. It has to be sucked up into the airbrush to mix, so it requires more air pressure to paint
(the opposite is)
Gravity fed -- paint feeds into the top of the airbrush from a cup on top. this requires less air pressure and less paint and can give you more control.
So now you can be an informed consumer!
Personally, I really like Badger airbrushes, although I haven't tried others. They're solid, reliable mid-range airbrushes. Parts are readily available and they're very easy to break down for cleaning. For example:
And you DO need to clean them. EVERY TIME YOU USE THEM. Make SURE she understands this. They aren't just brushes like paintbrushes, they're tools, and are designed pretty precisely-- if you let paint dry in them they become useless (and much harder to clean). I can always tell when I didn't clean mine well enough because I start getting spitting and weird issues like paint not coming out with just a little pressure, requiring a LOT of pressure, with problems and stuff. I usually clean mine with q-tips and rubbing alcohol-- if she only paints with acrylic, clean-up can be very easy. If she decides to use enamel for some reason, she'll need paint thinner. Stick with acrylic, I do.
So, my Badger 155 lets me do stuff like this:
That's pretty much all airbrushed-- it's about the size of the palm of my hand. For a good feel for the detail you can get, I initally painted the wheels all black and then airbrushed the middle parts with green and brown, without touching the tires. The camo is freehand airbrushing with some 20/0 brushwork for the tools and some details. And that's with a midrange airbrush with a mid-size spray tip and needle...
So, with all that in mind, here is what I would recommend:
A Badger Anthem 155 airbrush. A set like this: http://www.amazon.com/Badger-Air-Brush- ... B000BRH58C
That includes a cup, a couple of bottles, a hose, and some cleaner. (You don't NEED official airbrush cleaner-- get a bottle of rubbing alcohol at your local drug store. If you're using acrylics, that and water and q-tips will take care of it.) It's a pretty good deal. You will need one other thing:
-- a regulator with a moisture trap. this is pretty vital, depending on what your shop compressor has. If it already has a regulator and a moisture trap, you're good-- you just may need an adapter to hook the airbrush hose to it. The regulator should be able to provide between 10 and 30psi-- higher pressure does weird things like "spidering" and can be very hard to paint with. I usually paint at 15-20psi.
That's going to be something like this: http://www.amazon.com/Airbrush-Compress ... B00171BFKK
A second option for a gravity-fed airbrush would be this:http://www.amazon.com/Badger-Air-Brush- ... B002W84GTO
Or if you feel like spoiling her, this: http://www.amazon.com/Badger-Air-Brush- ... B0078MEXX8
...but if you get her that don't tell me or I'll get all jealous. That's my Christmas present for me if I get a good bonus for work this year... 8)
There are other useful things-- depending on what she wants to paint, art stores often have basic sets of acrylic paints. It may be helpful to get a bottle of retarder or airbrush paint medium, but that gets into some more complicated mixing and stuff. I kinda love it, but I'm not the target audience here. (I can recommend paint for plastic models-- Vallejo Air Color is very nice and premixed to the right consistency for airbrushing-- but I suspect that's not what she wants to paint.)
Avoid at all cost cheapo airbrushes-- Testors Model Master are really bad, cheap plastic junk. Iwata is pretty fancy. Paasche is good, but I think a step down from Badger. There are also very inexpensive clones of big-name airbrushes, mostly Chinese and mostly under $30. I have heard such mixed things about them that I wouldn't recommend them at all. Airbrushes are also often deeply discounted. If you find something you're looking for, shop around and see what you can find. These guys: http://shop.webairbrushes.com/
-- have very good customer service, but their prices are slightly higher than people who sell on Amazon. (They're the retail end for Badger, I think.)
I think that probably covers more than you wanted to know... If you have specific questions, ask! I can either answer or I can find out. I seriously love airbrushing. It's weird, because I'm not much of a brush painter and I'm really not a visual artist at all, but give me an airbrush and a plastic tank and I can be happy for hours. I even enjoy the breakdown and cleaning part, which is probably really weird. 8)