CNC Colchester chipmaster

[m_c]

Unless you're going to replace the v-belts with a toothed belt, a closed loop motor isn't going to be much benefit, as the belt will cause slight speed fluctuations as load changes.

You also need to consider gearing, and how much torque you're likely to need at the spindle. Are you going to continue to use the gears, if so you need to factor that in, at which point you need to use a spindle encoder, as you'll struggle to get threading to work in multiple gears without a spindle encoder.

Off the top of my head, the following motion controllers spring to mind-

PoKeys 57CNC something or other (I think it supports encoders)
PMDX have a couple motion controllers, but they're Mach 4 only, and you'd need to check they support lathe (I think they do, but not 100% sure)
CS-Labs CSMIO/IP-A or S along with their threading module.
Dynomotion KFlop either on it's own, or with any of their add-on boards (Mach 3 only, or use their own KMotionCNC software)
SmoothSteppper, however I'm not sure if they have implemented full encoder for the spindle yet.

There are also the UC100 and 300, however I'm not sure if they support turn/threading.
A key thing to consider, is if the boards support turn/threading. Not all boards do.