Hi Dick,
you could use the KB controller you already have with a 90 / 180v DC motor, with the correct "power resistor" (which is in the motor return line, and senses the motor current) or, as you suggest, a 12v motor fed by one of the inexpensive EvilBay PWM DC motor controllers fed in turn from a transformer and bridge rectifier. A transformer for e.g. 1/4 HP at 12v needs to be able to deliver at least 15 Amps, so won't be small or particularly cheap - e.g. my suggestion of a thrift-shop start & charge battery charger (most of which will be happy delivering 15 Amps or more continuously - I use mine for allsorts, including the E.G.B.E.R.T. (Electrolytic Gungey Bubbling Encrustation Removal Tank - electrolytic rust removal!) with a 20A MCB on its transformer output - cheap way to deliver fairly hefty currents at 12v-ish.
I'm also planning a T&C grinder, but I have a 1/2 HP 180v DC motor squirelled away alongside one of the KBIC controllers, most of the DC motors of that type (e.g. those used in minilathes etc.) will run up to 5 or 6,000 RPM and down to a few hundred* so should suit a small T&C grinder pretty well in direct drive?
I wouldn't be so concerned with the motor bogging with a larger motor pulley, the inertia of a Quorn or similar spindle isn't all that much - the 1/2 HP motors on minilathes etc. cope with a much heftier spindle and chuck plus workpiece, if you have to wait a second or three for a T&C spindle to come up to speed is it a big deal? Anyway, if the motor can do the RPM you need, you shouldn't need the pulleys to speed it up much? Higher speeds will have the advantage of better fan cooling too!
Dave H. (the other one)
*it's worth adding an external fan, e.g. a PC cooling fan, if you'll be running at low RPM a lot, they tend to let the magic smoke out if they overheat! I like cooling fans, my rather-larger lathe has one on its motor so I can run it at 5Hz and get *incredibly* low speeds:
View attachment 139313
That's a 10 inch EBM Papst axial fan, yes, the motor's trashcan-size...