Before you can make a dicision on how to vary the speed of your spindle you would need to know if the motor is single phase or 3 phase. What kind of control does it have now? What are the speed limits you are looking for.
Using a frequency drive can be expensive and you would need a three phase motor. there are products in the market that will change Pulse width (Mach3) to voltage.
PM me if you want more info if you wish