The type of material you want to cut has no influence on the torque that your steppermotors must deliver. The force to cut into the material should all come from the motor that drives the router bit. The torque of the steppermotors have influence on the acceleration that the movement of the head can make. You will have to determine what is the weight of the head assembly (for x, y, z axes, don't forget gravity for the z axis), and calculate how much torque you need to accelarate that weight in that axe direction.