It depends how hard you are going to work it, whether you need it or not. I put a 7.5hp motor on a small knee mill so that it would have low end torque for tapping. Speed control was via VFD in this case.
Why are you going to DC? A spindle motor makes a lot of turns, compared to a servo driving a table screw. Brush wear and maintenance is something to consider. A DC controller is likely at least as expensive as a VFD for a 3 phase motor. You can run one of these from a single phase supply, providing you oversize the VFD by 50% if you want full nameplate hp out of your motor.
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