well come to the forums and i hope this answers your questions:
1. with 12 cells i would go with a 9L so you can always add a few extra if you want and the 8L will run kinda hot.
2. i would start with a 10 or 12.
3. you dont NEED a ubec but its a good investment.
4. with gp 3300s i would expect about 12 to 15 minutes.
3)The uBEC would be a very good investment, especially if you decide to upgrade to 14cells later. The 9920 BEC only works for upto 12cells, and even on 12 it might be a bit dodgy.