Quote :-
"I want some extra power from the lathe spindle, and also some extra speed, and just 'control' of it would be o.k i.e spindle on/off auto, and rather than building a relay box, id want mach 3 to control it all with pulses. the standard motor HP is around 1. i want maybe 2hp/1.5kw which is a lot for a tiny lathe!.. but should see me good for parting steel with ease."
I would suggest a VFD and matching motor as a package. 3 phase is smooth power! From zero to twice motor rated speed, easy fwd/rev and speed control. You may need speed reduction pulleys for high torque at low speed. Polly V belts are smoother, with less vibration than toothed belts and you can make your own custom pullleys with a suitable die box chaser or V tool on a lathe.
Ian