As long as the stepper is perpendicular to the screw flex couplers are not that important. If you use mounting screws that are slightly smaller than the holes in the stepper motor, that will allow you to align the stepper motor shaft to the screw.
Acme screw verses threaded rod will be better.
The least expensive is Enco 1/2-10
Dumpster makes a inexpensive anti backlash nut.
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