I recommend checking on the RR2 forums over at www.gametechmods.com. There are lots of people over there who have been building some great heavyweight robot replicas.
In general, start with Ampflow A40-300 motors, attach some wheels, and be sure to increase the armor plate thickness in the “Materials” tab so that your robot weighs at least 100 kg. Otherwise the motors are so powerful that they will flip your robot.