Help clear some things up

Ok, I just bought myself a Kingwin AquaStar AS3000 Liquid Cooling Kit (go ahead, beat me up for buying a kit) and I have a few Q's over it

I know you guys said to steer away from kits, but let me explain myself. First, this will be my first liquid cooling project, ever. I bought this kit based on everyone saying this was the easiest kit to install, plus, I'm not an overclocker (Running Operton 148 and Radeon X800), so I don't need anything top-of-line. I'm just doing this to do it, and to stop the noise coming from my PC since it sounds like a jet.

1) I always hear distilled water when talking about liquid cooling. Do you need to mix distilled water with the coolant, or is distilled water a coolant itself?

2) I was looking at some coolants at FrozenCPU.com. Which do you recommend for me? Is the $37 coolant really worth it?

3) On average, how often do you need to change to coolant? When changing it, do you need to remove all tubing to make sure you get all of the coolant out when putting new coolant in?

4) How do I determine what size tubing I should use?

Thanks for your help!

Jordan