I have a few old tobacco barns on my farm that I use for storage...I'll post some pics later if you want.
I used 3/4in rock to cover the dirt to keep the barns cleaner, but that is it...it is a terrible surface otherwise; great for storage, horrible for anything else.
One option would be to frame in a raised platform. That is very common in Maryland in old tobacco barns. They called them, "stripping rooms" because that is where they stripped the tobacco from the stems in the winter. I have used the stripping room as a workshop for the last 14 years
Mine has windows...in the summer I put in a window air conditioner and in summer and use propane heat in the winter.
All that said, I plan on building a real workshop this fall...concrete floor, the works.