I bought a set of three FL57STH76 stepper motors from motion control products . These are hybrid stepper motors with 8 wires and 200 steps per revolution. The rated voltage of these is 2.0V at 4.0A per phase when the coils are wired bipolar in parallel. This configuration should give a holding torque of 1.85 Nm (or around 260 oz-in).
Attaching the motors to the leadscrews
The motors drive the leadscrews directly, so they need to be attached using some sort of coupler. Lovejoy type couplers use interlocking jaws with a plastic insert to connect a pair of opposing shafts. This allows for some missalignment of the shafts (which is very likely in our case).
The motors are mounted on rectangular pieces of HDPE plastic (0.75 inch thick kitchen chopping board). This arrangement allows the couplers to be attached outside the frame of the machine, and for the leadscrews to be bolted onto the bearings. This puts some tension in the leadscrews and hopefully reduces whip in the leadscrews.
I bought some couplers from fish4parts which seemed good value at around £2.50 per set. They came with pilot holes only, so they had to be drilled to the correct size: 1/4 inch for the motor shafts and 8mm for the lead screws. Getting 1/4 inch sized drill bits was a bit of a problem. I also had to drill and tap some holes for M3 grub screws to hold the couplers onto the shafts.