PS outside of case and how much for a lot of fans.

First of all, this isn't for a computer.. it's to rig my media shelf for better cooling. My media center has closed in the back so all the heat from my AV equipment just builds up in their compartments since there isn't much room for escape quickly.

I am planning to use PC case fans to vent out hot air from the back and power it with a regular PC PS. I have about 4 shelves and plan to use a total of 8 120mm fans (2 for each).

Three questions:
1. Will I need to worry about anything other than watts of the PS?
2. Is there a way to turn them on/off? No point in running all of them when I only need to run the fans in the compartments that need cooling.
3. Is it safe to leave the PS outside of the case? All this will consist of is a PS, 8 fans, and my media center (which is made of that cheap wood stuff).

Yeah, I did a bit of thinking and figured that one good fan should be enough to supply an exhaust to each section.

It seems that the best would to NOT use a PS and the alternative seems like a pretty viable idea.

Two question/concern with this part...

Each wire from the fan going to the power cord right? (Just making sure as this wasn't something I planned to do.) But how do I know which side is + or - on the power cord?