Intresting.
What do you think of gears and DC-motor?
That small motor and wide diameter of drills/mills and materials needs consideration on rpm.
I have been toying with same basic concept but with poly-v, AC-motor and VFD. It turned out a bit heavy. 0,18 kW 3000 rpm motor is about 3,2 - 4 kg. Also tried a little 200w AC servo (half the size and quarter of the weight), but it needs a servodrive, it is not stable enough with my VFD.
Skip few years....now there are DC BLD motors and drives. I bough one 48V and 200W or something. I haven't bolted it on anything, just tried it out of the box and it definately has something on it. Some years ago I calculated typical mill/drill speeds for steel and AL for 3-6 mm diameter. And concluded that hight end close to 2000-4000 is desirable, but not that critical really.
Where problem lies is torque at the 5/6 mm drill and mills, have to gear down to 400 to 600 rpm while motor produces top power (i.e. close to nominal rpm). This is where normal AC motor excels, 4kg 0,18kW motor can be abused short time twice the torque at nominal rpm and only problem with quality motor is that it might inflict burn on your arm, whereas other motors cut out or start smelling funny.
Original step mill is a self contained unit, all controls on it. Any modern motor needs a separate unit for power/controller, depends on personal preference. I like it, but I see if anyone has different arguments. I like low voltage and well insulated stuff.
Not sure if this personal rambling is of any use, but use what you like, and critisize rest of it.
Pekka