If it were me and it's not..   I'd dig around the posts, cut them off at ground level bolt on steel i-beams on the bottoms,  weld some iron sticking out toward the bottom fill the hole with concrete.  Make the I-beams long enough so with finished floor there is 20 feet clear between floor and ceiling,  and go from there with finishing off. (insulation, electrical, etc.. )    Put in the conduits etc.. needed under concrete floor before pouring.

If the current building is setting dead-flat with the surrounding ground,, I'd seriously consider building up the bottom so the floor is a foot or so above surrounding area.

Yah it would take jacking up the building all at once while putting in the steel I-beams but thats not insurmountable.

Far as average size of shop per acre,, not very meaningful at all.  Knew a farmer who didn't have much of a shop at all,,, He just paid the local repair shops a lot more money that most....   More accurate is will anything owned fit with enough room  for another machine or 2 ? Then add 40 percent onto that. 



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