If you can afford an 8 N.m stepper motor then you should be looking at direct drive (no annoying backlash
).
What is the phase current of the stepper motor and what voltage are you intending to run the stepper motor driver from?
Supply voltage x Phase current give the nominal power of the stepper motor in Watts.
The speed could be limited by the stepper motor power (Velocity (m/s) x Force (N) = Power (W))
What is your spindle power and maximum spindle speed?
This should give us an idea of the maximum force required (to drive the cutting bit through the wood).
What is the expected weight of the gantry in kg?
The maximum force should be the cutting bit but moving the gantry could be the limiting factor.
What sort of accuracy are you looking for?
Recommend aiming for 0.01 mm/step, any smaller can limit the maximum speed (try and keep it below 50,000 steps/second)