mklb,
My first concern is with your spindle motor, an a.c. blower motor. They are pretty much designed to to run at one speed or two ( heating and cooling). The torque will drop off as your speed is reduced. A step up would be a d.c. servo motor with a drive or a pemanent magnet d.c. motor with a pulse-width modulated speed control. With a d.c. motor, you will have more torque available over a broader speed range. Years ago I had a Taig lathe that I mounted/ used a Dayton 90 volt d.c. motor. The speed was controlled by a lamp dimmer driving a bridge rectifier/ capicitor to convert it to d.c. Very simple set-up.
If it's not nailed down, it's mine.
If I can pry it loose, it's not nailed down.