Needs must, Stonemason from Scotland

[JAZZCNC]

First Rack n pinion will be perfectly fine and actually preferred for this length machine.

Your requirements of 0.5mm are very achievable but still the machine will need to be strong and correct regards the choice of linear bearings.
I helped a friend build a machine to cut granite worktops and it's design was so simple you wouldn't believe.? It was made from RSJ, boxsection, 4 x stainless flat plates and Concrete blocks.!

Concrete block walls where built to height. 2 x Stainless plates was drilled and set into the top of wall with threaded rod concreted into walls. This plate had series of holes drilled and tapped all along it's length at 100mm centres down each side. These holes held short stainless threaded rod with lock nuts then a matching stainless top plate had the profiled linear rails and Rack fastened to it. The top plate was then set flat and straight by adjusting the nuts on the threaded rod and locked with the nuts.

The gantry was a 12" RSJ braced and stiffened with plates with profiled linear rails set and shimmed on it's top & bottom surfaces. The Z axis was a Massive piece of 18" Boxsection that used to hold up a petrol station forecourt canopy. Into this was set the spindle off a Stone cutting machine which my friend already had. This was very heavy and counter balanced at the rear with pulleys and counter weight running on cheap round rails.

The bed was just a big table made from steel boxsection frame that pivoted at one side and powered with hydraulic rams off a fork lift to make loading big sheets of granite easy, the frame was again mounted on concrete block base.

It Worked a treat right upto him going bust then it was broke up and sold for bits. This was the only down side because it was really a permanent fixture.!!

3 RSJ's would make a very easy build and could be made resonably movable.

It used 12Nm Stepper Nema 32 motors using a 5:1 gearbox running and 240v drives for X and Y and I think the Rack was Module 2. The Z axis was 20mmDia 5mm pitch ballscrew using 2:1 ratio and 8Nm motor.

After many weeks of tweaking it easily held tolerences close to wood router and far better than expected. Speed didn't matter because cutting stone is slow process anyway.

What I would say and advise is using decent rails and get them up high out the way and cover up has much has possible. Don't think I'd go with V style bearings or any home brew has this will be area that gets punished the most from the constant water and grit.
Same goes for the Rack n Pinion get it out the way around the back or down the sides and cover much has possible.

Hope this helps.