Hi,
its been a whaile since I've used Mach3 but certainly Mach4 has the option of a step/direction spindle, and if memory serves I think
Mach3 does as well.
I do agree with the previous poster, steppers just plain run out of steam the faster they go. A step/direction servo is a good choice though.
I have a 1.8kW 3500rpm 6.2Nm (cont) Allen Bradley AC servo and drive as a spindle motor and it kicks arrsse!
Craig