As Matt pointed out...get a Wattmeter. That is the single most important peice of equipment for electrics. It can save you hundereds.
The setup you have is a good one. As a matter of fact...they are all good as the possibilities for gearing, propping, and powereing are almost endless.

The 4100 geared 6:1 on a 3S and 12X6 should yield about 13 amp WOT. I would stick with the 1320 Thunderpower batteries as they are perfect for this app.

Glad to hear it worked out for you and your friend. And as Matt said...ask here first and dont rely on the guys at the Hobby shops. 99% of them only know what they read on the side of the box. There is real world data in the forums.

-Mike