Looking to build another cnc specificially for solid surface carving

[routerdriver]

I would suggest that the first two questions are asking the same thing in a roundabout way.It comes down to how many steps per mm and a finer pitch with no gears will move the same distance as a coarser pitch with a gear reduction interposed.So perhaps check the prices of a couple of thread pitches for the lengths you require and of sufficient diameter and use this as a basis for your decision.