For wheels geared Motors of 6, 12 and 24 volts are available from 35 RPM to 1000 RPM which can be used to drive the wheels. Select the one with which you get optimum speed with the balance of your robot so that it doesnt topple at full speed. There is no direct relation with whatever voltage you use. May be 36 or 6. It all depend on what motors you use with the said voltage. For weapons you may use servo motors which have precise rotation with speed and torque.
Cheers and good luck.