First off, seeing as this is your first post to FSM, welcome to the forums.
OK ... now for your questions
1) Not really .... you'll go through 1 or 2 cans per model anyway. A compressor will supply you with a constant and controlled flow of air where as the cans don't.
2) Depending on where you live (Canada, U.S., somewhere else). You can pick up a single action Paasche Airbrush for $69 in some locations in the U.S. Add to that a compressor from Wal-Mart or any good hardware store for between $99 and $129. By using a small industrial compressor you have the option of more uses around the house while a hobby compressor is pretty much limited in it's usage. Final things to make sure you have are a regulator (which sometimes comes with an industrial compressor) and a moisture trap (very important and costs about $10).
3) Depends on what you want to paint. I only use a single action airbrush for painting my armour kits. I can achieve the camo looks that I want to do so I really haven't found a need for a dual action unit yet. Also the learning curve with a single action is much smaller.
Happy Modelling and God Bless
Robert