Does the mill have the Denford USB controller?
If it does, I would personally run with Denford's VR milling, as although pretty basic, it does work very well.

Even if you do upgrade, my preference would be individual stepper drivers.

Spindle wise to get 20-30K, you'd really need to remove the existing spindle, and look at fitting a water cooled Chinese spindle and VFD setup. It would most likely involve having to machine out the head casting to accommodate the spindle, unless you bolt it on the side.