A VS spindle is not necessarily capable of CNC operation, it depends on what operations you want to do.
Ideally the spindle should have good speed control regardless of load, this usually means feedback of some kind from motor to drive, if threading or CSF is required, an encoder is needed on the spindle.
If you buy used, make sure it is a make that the controller is still supported, e.g. Mitsubishi Mazak Fanuc Fagor etc. or at least the manuf is still in bus.
There is quite a bit mechanically to retro-fitting a manual m/c, ball screw, motor mounts etc.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.