The VTC4 if you want to go full throttle with it but I have had no issues using the HG2s and they are superb for battery life. I get the same as vpjoy stated above 1.5 to 2 days per charge.
Up to you. Put in the resistance of the build and what watts you want to run it at. Don't go over 20 amps with the batteries, they are in series, not parallel.