Since I am not there and cannot possibly see the condition of the shop. This means I can't really give you an opinion. There are many variables to deal with noone has brought up. The condition of the roof? does it leak? How is the ventilation? How big and drafty are the garage doors, how often do you go in and out?, Is there a vapor barrier under the slab of the concrete? What is the orientation of the building to solar gain...if any? These are just a start.
Do yourself a favor and get it right, hire a professional for a couple hundred dollars and have someone actually look at your project,it will be $ well spent to ward off potential issues down the road. And what to look for if you start to have problems.
for a start, they will look at your issue and give professional advice...on scene.
Good luck, we all want a warm shop.