Actually 1S Lipo will be lighter than 2S A123.
For Lipo, you will have to consider either 1Cell (3.7v) or 2Cells (7.4v).
The placement of batteries and choice of motor will be different for each setup.
Just a note: Lipo and A123 are not race legal ... this is just for fun (or testing purpose).
If you want to be legal, its better to use 4cell NiMh.
I also play 1:12 at TRC. I am using RC 12L4, Li-Po 7.4V and brushless.
Car : 12 L4
Battery : 7.4 V Byenergy 分体 4200mah 25C
ESC : EC power 120A with sensor
Motor : Dualsky 8.5T
Gear ratio : 96T/22T
Tire diameter : 48 mm
It may run for over 9 min, after run the Volt arround 3.84 each cell.