Z axis- Knee or splindle?

Hi all,
I'm in the process of converting my Chevilier turret mill to cnc and
I think I'll use the knee assembley as the z-axis, but before I
commit to that I thought I'd ask a couple of questions first. I think
what I'll need to do is use an air assist system rather than use a
large and/or substantially geared down servo to lift the knee
assembly, would I need a ballscrew with a system like this or would
there be enough weight over the knee to take up the play? Would
servos the same torque as that I would use for the x and y axies be
siutable? Is there anything else I should know about this type of
system, or would someone point out the pros and cons of each type
(spindle v's knee)
Thanks
David Preston