I just posted about a conversion project like yours a few days ago (last Saturday?) on the Grizzly board. I guess I should have put it here.
I'm also doing the phase 1 from Hoss, with the big difference being I cut all the metal parts first. I don't have a 3D printer, but I have a CNC Sherline(/A2Z CNC) system. I cut everything on that.
But your motors, breakout board from CNC4PC and the rest sure look very familiar. Tonight I was going over how to wire up everything so the limit switches and the Big Red Switch work with it. And I think I'm on the way to eBay to look at those ballscrews.
My Sherline has the A2Z upgraded XY base. The leadscrews are 4TPI, Teflon coated Kerk screws. They do have a few thou of backlash, though. I was also thinking of looking for other Kerk products.
Bob