The best advice I can give you is this... Separate the mounting of the lead screw, and the driving of the lead screw, as much as possible.
Here's what I mean...
Take your axis, and install your lead screw. Now, it should be completely self supporting, and tensioned between appropriate bearings. You can actually do it with a bearing on only one end, but this is a less than ideal solution, because it places loads on the bearings inside the motor...which are not designed for that sort of loading.
Now that you have a self-supporting, self contained system...connect your motor to the end of the screw. This can be with any sort of coupler you want...from the high dollar helical couplers to the lowly air-hose couplers, or even througha pulley or gear arrangement. The point is that it's a separate step. NO loading is put on the motor, and it lasts longer that way. PLUS you can tension your lead screw, to reduce whipping at higher speeds.
-- Chuck Knight