I agree with Gerry.
I am not sure exactly what size steppers to get but it is very common to oversize them and loose performance. Case in point many Taig conversions come with 425oz motors and a 24v power supply. Motors are running way below thier proper voltage and then they are also run on a fine pitch screw. Dropping to a 200oz low inductance motor makes the machine run much faster in the rapids and it still has plenty of torque for the cutting.

On my router for the Z axis I use a 425oz nema 23 with IIRC a 5tpi ball screw, G203V and ~70v and I have run it over 500 ipm. In my never to be humble opinion it is very important to get the Z axis setup correctly. You can loose a tremendous amount of time when drilling holes, especially when peck drilling if the Z rapids are too slow.

Mike