Tim, first, radiant floor heating is nice, but you have to be very careful with it. You need some sort of emergency back up heat in case the water heater fails, or if it fails in the winter, you will wake up to a frozen line. That may not cause too much damage, or it could split your floor open. That being said, I know a lot of people who swear by it, and have never had any trouble with them....I would at the very least, have a back up water heater ready to hook in, just in case.
Second, you need to ask your zoning board about a variance to run your shop from the property. They will want to know things like volume of traffic you experience at your present location, and will want to know that excessive noise won't be an issue. If you live in a very private gated community, you can bet it will not happen. But if you live in a small town they might actual encourage it. You must sell them on the idea of your business adding to the community.
I know a lot of people who have built shops, some all metal, some combination, and some mostly wood. The all metal freestanding structures are great, but as said, they will "rain" inside if left uninsulated. Spray on foam is likely the best fix for this, but being you are working with metal, there is a certain flammability involved. I personally built a pole barn, made studded walls between the poles and insulated like a normal house, and I could not be happier with the result. I also used OSB on the interior, which seems to resist most grinding sparks, but I do chase all the hardy welding sparks, as they might be hot enough to ignite the wood. Building codes may play into what is required in your area. And the all wood buildings are the easiest to build, but require the most maintenance. I would also add that a buddy of mine followed my design, but instead of OSB, he went to a used garage door auction, and bought enough insulated garage door panels to line his walls from floor to ceiling, and it worked very well.