Hi John
If i had your size of mill it i would convert it to CNC ,,,,,,, most lads buy a hobby mill learn to use it then decide to convert it to CNC ,, space is a big factor with with larger machines .
Bigger machines ,,,,,, larger cost to convert to CNC
Ballscrews are desirable to eliminate backlash and are allot smoother in operation ,,,,,,, Mach3 dose have an anti backlash compensation feature ,were you program in the backlash of your screw ,,, suppose this feature is only going to work if the screw is new and is not worn more in the middle section of the screw were most work is done on the table . Also having backlash in the system causes problems as the tool path changes direction , causing tool dig in or breakage.
Hope fully someone with more CNC experience will chip in .
Rob