garage heating

What about natural gas ? I bought a used 60,000 BTU furnace from a house that was being torn down for 100$ along with all the plumbing and vent pipe I wanted. Furnace was 2 years old. Trenched the yard from the house to garage, laid the pipe, installed the furnace and never looked back. I also insulated the heated portion of the garage. I can raise the temp from 50 F to more than 70F in a matter of minutes. I keep the garage at the min temp level just to keep condensation etc down on my cars, helps keep the polished metals nice and corrosion free.

If natural gas isnt an option and you dont intend to maintain heat at a minimal level, then I would strongly consider a wood burning stove, set a fan near it to circulate the heat and it should work pretty well. Depends on how cold it gets by you.

Insulation is the biggest thing you can do to improve the heating, along with making the space as tight as possible to un heated air.

Propane would be good too, if you can use a vented type heater, and not one of those torpedo things. The torpedo heaters are mainly for construction work sites where theres lots of cold air entering the space intended to be heated.