I imagine the mill has steppers so the easiest way would likely be to replace the drives but keep the steppers if they are ok. Have a look at the Gecko G540, it might be a cost effective drive setup for you and Geckos make, in my opinion, excellent stepper drives.
Spindle I am not sure of, just depends what it is, if it just needs a 0 to 10v input then possibly the G540's spindle output would work.
Hood