Save your money, M's aren't meant to be run at more than 5-600 rpm anyway
The backgear gives you huge amounts of torque for milling etc and a standard 0.3 or 0.5HP motor will give you various speed options
Get a standard M setup to start with, then see if the expense is worth the effort
The standard setup can do amazing amounts of work with a decent constant rated motor, working hard 8-10 hours a day kinda stuff, it's a proper semi professional machine
I reckon that some of the milling sessions I did in my early days would have fried anything with electronics