Following Dean's drawings I finalized the base design, so any comments and corrections before I start cutting would be appreciated.
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Blue steel (top rails) is 80 x 80 x 5.
Red steel is 60 x 60 x 5.
Green steel is 60 x 60 x 3. (trying to save on weight).

Top rails would be bolted to the supports for alignment purposes and use epoxy putty(later I would need some more details on that) and then epoxy leveling. Final length of the top rails (overhang) to be decided along the process.

What height from ground to top rails is recommended?

I ordered a straight edge 2000mm DIN 874/2 from same place Boyan did, thank you Boyan for posting the link,
and I also have a precision level 0.02mm/m so I hope I will have a quite true reference plane to start.

Thank you