Garage Heat?

Based on the size of the shop. you will need approx. 40,000 btu's to do the trick.
I would try to find a sealed combustion gas furnace. Add some duct and a cold air return at the floor. A sealed combustion unit will not suck in outside air to use for combustion and the furnace could be as high as 95% EFF. Spend you fuel savings on moparts. Good luck with the heat, as we all know this winter has been too long and cold.