I used a product called slab shield under my floors with radiant tube heat. It is a lot like bubble wrap insulation but has a foil layer as well as 2 flexible foam layers. It is only 3/8 thick and does the job. As for the concrete thickness I would think you want at least 10 inches on the main drive path with minimal stone under it. I used wire and fiberglass reinforced concrete and have not had any breakage but my weights are more like 20,000 not 110,000. I did my center section as a separate slab and went 6 inch or more on it. You could also run the radiant around the rest of the shop and not do the center where the heavy weight will be concentrated. I strongly advise sleeving the tube where it passes from slab to slab with a bigger diameter plastic conduit. This will prevent pinching the pex as the slabs shift.