In practice you really want the e-stop to cut power to everything. And then there's always the problem of machine startup and shutdown -- I guarantee you'll forget to apply the brake or park the z at its lowest setting. From experience, (we had a high-lead ball screw that would backdrive under the weight of the Z), I would make this axis screw driven. You can still get high speeds, but without the worry.
Ahren
www.cncrouterparts.com