130-150 mm on the z is a good range.
Use the same screw for the Z that is on the y.
Gecko g203v All sorts of oops protection and the morphing will give better top speeds which can be very helpful with plasma on thin materials.
75v power supply and steppers to match. High voltage means better torque at max speed than lower voltage.
www.candcnc.com for breakoutboards and torch height control.
or possibly a complete electronics package.
Make sure you have a great vacuum when using the router, the dust from mdf or many other things isn't any good for you lungs.
Check around but I have heard some speeds for plasma on thin material that are very fast. Your plasma user manual may give you an idea on how fast you have to go and make sure your screws and motors will go that fast but I think you are on the right track.
I can't wait to see the drawings.