For typical operations such as straight cutting, slotting, drilling, etc, the Z axis moves so minimally compared to the other two axes that it probably wouldn't matter.
On the otherhand, if you frequently make 3D parts where the Z is constantly bobbing up and down, I would convert both the column AND the quill to CNC.
One could argue that it'd be simpler and cost less to just do the quill, but IME, the column leadscrew that comes with the machine is a piece of crap with an enormous amount of backlash. You want to replace that thing no matter what, so you might as well spend a little bit more to get that thing motorized.
Likewise, it'd probably cost the same to convert the quill as it would be to add a counterweight/gas piston assembly. It'd also be prettier. I've seen those X3 guys add weird cable pulley systems and it looks like a circus rig.
