With your parameters, I guess your robot will be about 5kg to 15kg weight?
You may simply use DC motors with gear box.
As the reponse of a motor, there is a linear range between applied voltage and velocity. You should run your robot within this range. You may visit Maxon motor page (pls search on web, I don't remember). They will help you to compute motors' characters, and send you the suitable datasheets via email for free.
I think you should use 60W to 90W DC motors. If you want to use high level controller (realtime), you may use it with encoders. In a close loop, you don't have to worry about if the motor is linear or nonlinear.