I have a 2 car garage that I heat with a 30,000 btu (240 volt) electric heater. The garage doesn't have any insulation and 2 drafty doors and a couple of woodpecker holes, but it does fine. My big garages, I used a salamander kerosene (turbo). I have a small one and a big one. The issue with these, other than a 3 foot flame coming out of them, is that they raise the temp quite fast with causes condensation to form on large metal parts like engines, the parts are still cold while the room is warm and moist. Propane is good if you already have propane, but the by product of the combustion of propane is H20 (water), so things get wet and rust, the amount would depend on the degree of air tightness of the garage. I do have a buddy, machinist for NASA, that has a small room in his barn that he heat with propane and he complains about the water, but the heater works great. Also I bought some propane heaters from Northern Hydrolic on sale for half priced a while back. Never installed them , never got propane.
It is not the cheap way, but it would be the best way and that would be to install a self contained wall mounted heat pump. Heat pumps heat slowly so the condensation won't build up
1985 CJ7 - total customization underway, Sublime green and Plum crazy purple, Ford 9", 258 w/ Holley 475
1985,93 CJ7,YJ - L98 TPI Vette motor, Atlas twin stick, Dana 60s front and rear, Arbs, 39.5 Swampers