I'd have to agree that a dual action airbrush will be your best bet just because of its flexibility in what you can do with it. A single action airbrush is good for wide areas of one uniform color but that's about it.
When it comes to brands of airbrush, you will seldomly go wrong if you purchase products from Paasche, Badger or Binks. I've heard good things about Iwata but I've never tried their products so I can't say for sure. I'd stay away from the Aztec line though, They are a fair product but I find even their dual action brushes are rather limited in abilities, they're not something you want if your looking at an airbrush as a long term investment.
Ventilation is also key, I can't accomodate a nice spray booth where I currently live, so I do all my airbrushing outside on my patio. You should also invest in a good quality respirator mask, the rubber type that seals to your face and has replaceable filter cartriges.
Don't be fooled into thinking that acrylic paints are totally safe just because they wash up with water or it says "non toxic" on their labels. Acrylics are plastic in a liquid form, this means that when they have dried, they are a plastic coating on whatever you applied them to, that includes the inside of your lungs if you inadvertantly inhale acrylic paint mist from an airbrushing session. Ventilation is good, protection is better still.
One final note about airbrush purchasing, if you're looking at Paasche products, You may come across an airbrush called the Paasche Turbo AB. This monster is unlike any other airbrush out there and is intended for the proffesional airbrush artist, not Joe Average hobbyist. Its expensive, very touchy and if you damage it, you need a proffesional to fix it. If you see one, steer clear of it. Its a fine tool, but not meant for us hobby types
Good luck!