Typically, all you should need is a deck, wiring harness, and the dash kit. Other little things are dependant on what you want to do.

"As cheaply as possible" is all relative. The dash kit and wiring harness are fairly "standard prices." About $15 - 20 for the harness and the dash kit about $30, give or take (the one I just bought was $25).

The deck can go anywhere from $100 (for a cheapie Sony or whatnot) to 4 or $500. Depends on the features you want, etc, etc.

Figure out your budget beforehand and then start to look. Thats your best bet.