To cut threads on a CNC lathe you don't need the gear box just a speed tacho on the spindle so the z axis can be synchronized with the main spindle. this means that almost any lead / ball screw pitch can be selected though if you want to run in manual mode then i would suggest that you stick with the same or integer multiple of the orginal which would result in the overall reduction of gear pitches cutable but some would be available.
as for a list.
CNC controller (PC, Software (Mach3), Breakout board)
2 compatable motors
2 methods of mounting them on the machine
method of reducing backlash on the axis
speed tacho CNC4PC do a good index pulse unit. Many used for threading and feed per rev mode.