Slap? Don't you love making these posts with no spell checker. I blame my spelling on the impairment to my pronounciation from having tounge firmly planted in cheek.
Is your older workshop is a pole structure rather than a conventional stemwall foundation? How much clearance to grade? Is the garage attached or detached from the house? Answers to these questions could change the following:
If you have a dirt floor, you can probably dig out enough to get the 3" minimum concrete thickness you will need for the shop rather than lifting the building. Don't forget you can rent jackhammers or power shovels to go through tough rock or soil. If you have any very large machines, you may want 4" slab. You might want to consider putting in a drain system and sloping the floor just in case you get water, and to make cleaning easier.