If you tear out the old floor, and replace it, 4 inch thick is fine,
more than that is over kill. Rather than the mesh, go with #4 (1/2
inch) rebar about 24 to 30 inch on center. Mesh is a royal PITA, and
only works if it is suspended in the center of the slab which is almost
impossible to do unless you use the heavy sheets of it rather than the
rolls. The fibermesh works well in helping to prevent cracks. By
varying the thickness of the slab from 1 1/2 inches to 3 1/2 inches you
can guarantee cracking. If you want concrete for a floor, replacing
would be the best choice. Putting in a wood floor would be a good
alternative for your feet and back. You could also shim the machines
and benches to keep them level, and work with the uneven floor rather
than replace it.
robo hippy