If I were you I'd go for an 18.5V li-po and a relatively cheap pack would deliver current as well as a more expensive pack if you go for one with a slightly larger capacity (meaning each individual cell is under less strain to output the same power) e.g. http://www.hobbyking.com/hobbyking/stor ... _Pack.html

Btw if you want an approximation of the GR01's with the tentacle calculator select the Johnson ('Small' RMP) motor and a gear ratio of 36:1 then,

for 9.6V motor- Voltage Constant 1890rpm/Volt, 0.50oz-in/A
for 14.4V motor- Voltage Constant 1430rpm/Volt, 0.68oz-in/A

Be aware it's an approximation but it should give you some indication of speed/current draw. Friction coefficients are very likely to be below the 0.9 calculator default btw in an arena environment and it appears to assume the full robot mass is over the driven wheels.