You could use a pilot motor to basically make it a rotary converter.
As for a VFD, not a huge fan personally but they do work well enough as long as you realise what their limitations are. Lots of people think that you can basically set the spindle speed to whatever you want, whilst that is true the torque drops way off at low revs and on a lathe that is where you need the torque. You can of course use one gear on the headstock for high speeds and if needing low speeds then use another, you will then be able to vary speeds in these ranges, within reason.
Hood