Yes, more than 40V will be much better - so long as your drivers are rated for more?

You can't get 10mm pitch 20mm diameter screws from eBay (or at least that was the case next time I checked) so it looks like you'll have to use 16mm pitch. I wouldn't go for 25mm as the moment of inertia of the screw will be significantly greater which will affect the acceleration.

Remember that a 10mm pitch screw on the Z-axis will backdrive very easily, so you will almost certainly have to prop up the axis before switching off. I used 5mm pitch on the Z-axis, but with a pulley and gravity does cause the axis to drop when I switch off.

If you don't want go spend a lot on another stepper/driver (fair enough) then why not link the two ballscrews with a pulley on each and a timing belt. Ideally 3 or 4 pulleys with the motor in between so that belt stretch is even... easer than the cable method I think.