Although your loss may be 27K, I'm a little concerned about R3 in the slab. You're losing alot to the ground.
If you're only doing the workshop, and no snow melt, and you never shut it down, you probably don't need glycol.
Electric would be bad, a water heater awful, gas or propane would be better. I'd run a gas line and get the smallest properly matched gas boiler you can find. Properly install it with low return water protection.