What is the Total Cost of Conversion (Excluding the Mill)

[Muzzer]

FYI, "NEMA" simply means imperial (inch) dimensions in tenths of an inch, so a NEMA 17 motor has a 1.7" square face. Beyond that, it could be a stepper, a servo motor or even an induction motor. There are equivalent metric (IEC) descriptors but the motors are interchangeable eg an IEC 42mm motor will be a direct replacement for a NEMA 17 but be aware that the shaft diameter would most likely be a metric size.

You can use linear scales for feedback and they are arguably better than using the motor position, as they report axis position without backlash and spring in the ballscrew, belt etc. But most CNC controllers don't use closed loop positional control. You can do it with Centroid Acorn6 or Linuxcnc but the former isn't cheap and the latter pretty much requires a PhD in Linuxcnc (speaking from experience).

I have a lathe running Linuxcnc in closed loop with linear encoders and I wouldn't recommend it unless you don't mind getting very deep into the weeds. Open loop will deliver good accuracy on a conversion and fitting closed loop won't / can't magically transform the machine anyway - lipstick / pigs etc. I have another lathe and mill running Centroid Acorn and TBH they perform about as well as I could realistically hope.

You will need to fit half decent homing switches (and ideally limit switches) to each axis. Unless you have encoders that are kept alive when the machine is off, you will need to home the axes each time you power up. It's possible to use a servo motor or closed loop stepper drive to provide an index pulse when it is at its zero degree position after the home switch is tripped but really you would be chasing your tail at that point.