Hi
I have converted two mills, a minimill X2 and an Optimum BF30.
Using stepper motors is balancing act that really requires an idea of what feed rates you need on each axis.
My experience (hobby use) - using HSS tooling for steel / Ali / brass most cuts performed at 2" - 10" ipm. Rapid movements limited to 20ipm for safety.
I have always used direct drive to ball screws. The X2 used nema 23 motors to BF30 uses nema34 motors.
I suspect that the nema23 setup will be ok as long as you don't want very high feed rates. Remember that using belt reduction for more torque means that the stepper is turning faster for the same feed rate - the stepper motor looses torque at high rpm so you can chase your tail.
I used md542 drives but upgraded to Gecko drives - much smoother operation + ability to handle up to 80V. My motors run at 50v.
Personally, I would recommend LinuxCnc as the machine controller rather than Mach3. I won't go in to the pros / cons but I found it interesting to see that Tormach have moved to LinuxCnc as their machine controller.
Here's a link to my BF30 conversion...
http://www.cnczone.com/forums/bencht...4-cnc-cad.html