Hey Arthur,
The hardware of PRO and Elite Series is identical. The PRO now have the 30 mm drag chains of the Elite Series and are also delivered with the Z-20 “heavy” Z assembly. So they differ only from the CNC controller and its power supply, and the stepper motors. The Elite has photoelectric limit sensors while the stock PRO and Original Series rely on stall homing (but you can retrofit limit sensors on the PRO and Original Series too). Both PRO and Elite Series have 50 mm rails on X axis.
There is however no machinist (16" × 16") model on the PRO and Elite Series. The Original Series differ in that they have only 35 mm thick rails on X axis.
The torque required also directly depends on the ball screw pitch. With a flatter pitch your motors don’t need to have as much torque as with a steep pitch.