I would suggest the 1/2 10 5 start. I have the 1/2 8 2 start on my current JGRO. it works much better then the single start you mention above. I'm building the 4x4 Hybrid, and on it, I'm planning the 5 start as Joecnc2006 suggests in his design.
Part No. 99030A7041018 Carbon Steel Precision Acme Threaded Rod 1/2"-10 Acme Size, 2 Turns/Inch, 6'L, Rh ThreadIn stock at $55.77 Each
Increasing the starts will increase your low rpm top speed. Stepper motor's max power is at low RPMs.
You can get flange bearings from this place.
It should work on both the X and Y axis for sure.