1. As MattD said
2. Again as MattD said, but if you are only using Wi-Fi then I would switch of 3G, it seems to save some battery life. It definitely doesn't last as long as the first iPhone, perhaps due to the fact it has GPS (A-GPS) and 3G though.
3. If you are on Prepay then Data won't work (unless you change the APN - see unlockit.co.nz). If you are on postpaid see MattD's response.
4. To help with your decision here are my thoughts, but it is still for you to decide. For such a small amount more you can get twice the capacity. After having the 8GB, and filling it up quickly, the 16GB is a breath of freesh air. I have so much space. As for white or black, I chose white because of the finger prints on the back of the black one. Yes you get them on the white but they are not nearly as noticable.