Z-Axis CNC Control of Grizzly Minimill

I understand it is not straightforward to convert the quill feed of
the Grizzly Minimill to CNC. Could someone please explain in more
detail the reason for this? Is it because the whole motor moves up
and down and has to be counter balanced with springs? Why can't you
just hook the stepper motor up to the original quill handle shaft
with a timing belt like the X and Y axes?

Would Grizzly's next higher expense mill (Model G1005 1HP Mill/Drill)
be a better candidate for CNC conversion?

I'm fairly new at this so please forgive my naievte'.

Bruce
Seattle, WA