Hi Dave,
I, years ago, put cnc on my ancient Henry Milnes knee type mill. When it came to the z direction I reckoned that the weight of the table,cross slide and knee (probably 250 lbs or more), would take care of any backlash in in the set-up so that I connected a fairly hefty stepper motor with a 3/1 stepdown that has worked perfectly for the last 6 years. It would be a very strong tool and motor that would lift any backlash into the system using the stepper onto the original windup on the mill and has proved to be troublefree, saving the expence and work of installing a new screw. Gravity wins!
another Dave.