12t would be plenty of fun. I have my grip car running 15t brushed and for messing around on the street it's all the power I need.

The Yokomo is actually ones of the few shaft cars that can be CS'd, partially due to their drifting design and popularity in Japan, hence more option parts.

Yep, for drifting you want the rear to spin faster. As I recall, underdrive/overdrive is just the way the gears are changed. Underdrive makes the front spin slower whereas overdrive makes the rear spin faster.