The entire structure may not be larger than a 31 cm. You can use any material for the base, including plywood, machined aluminum, plastic, or Plexiglas. Erector or Meccano sets are good too. So are Legos. A round robot usually works better than a rectangular one.
Rectangular robots can get jammed into corners or walls.
Your robot will need a central computer or controller. Next year, we hope that the BrainStem will be widely used. Your robot will need motors and servos. For motors, anything that turns a shaft will work fine. You will have to play with weight, position, power, and speed.