I just went with a 6" thick pad and 4kpsi concrete through the entire shop. and used the steel meshing and re barb around the perimeter. It was a little more then the reg 4" cost but I also have a 10x54 mill and didn't want the weight if that also cracking the floor in conjunction of my lift.
I do agree with not putting wheels on the compressor but what I did with mine I took some 4x4 and made a pallet. Now when i need to move it I just grab my pallet jack jack it up, slid it out of the way to do cleaning. As of late i have been making alot of pallets for the shop stuff to make...
Out of curiosity have you gone down to the planning/zoning board about your build. Sometimes they can give a good suggestion on a contractor. Also you might want to check with a local engineer. When I had to rodo my garage (total nightmare with the county but that's what you get with a foreclose...
Around here I would just go with a 6" slab and re barb the perimeter then build your walls from there. NC has a ton of hard clay and a shallow frost line.
It's really all personal preference on how many gallon compressor you want. I can say this from many years of using air tools. you can never have enough air storage. Right now I have a 5hp 80 gallon 2 stage compressor and run it hard when using my sandblast cabinet, die grinders and spray guns...