What IR filter should I pick?

I'm looking forward to buy a 52mm IR filter for my 18-55mm Nikkor lens. The problem is: I don't know the difference between: 720 nm, 850 nm, 950 nm, and so on. Can anybody explain me the effect of quantity of "nm" whatever this is. I already have a UV filter and a simple mc protector.

UV filter will have to be replaced with the IR filter.

How do you plan on composing and focusing with the opaque IR filter on the cameras lens?

You do know that there's an IR cutoff filter on the cameras sensor, so it is not real sensitive to IR.

Look here for a better understanding of IR and digital imagery

http://www.lifepixel.com/IR.htm

The IR sensitivity is not your biggest issue, although near IR like 720 nm is best for general IR photography