I also would suggest a oil furnace. You already have the electricity available and oil furnaces are more easily moved if you want or need to move it around to another location,of course thats considering there is no trunk line and branch runs. Heat output can be whatever you need ( btu wise ) and CO can still be a danger with oil, it is something to keep a watch on in a shop setting. Of course flame and spark dangers are extremely dangerous in any shop. Any fossil fuel furnace in a shop will need maintenance and safety checks,,,,,every week to be safe, have a professional do it. Keep a clean oil,air- filter and nozzles in it and check oil quality often. I have a smaller metal garage and electric heat and a wood burner do OK, especially on a 40 deg. day when the sun is out for hours. Fossil fuel appliances in a paint shop can be as deadly as dynamite,,,follow all code and safety recommendations. One other note, ,, on sunny days I paint because of the added heat and with a furnace 'outside' of room air,meaning that it does not use garage room air for combustion air and is safely isolated from garage room air there is no danger of garage vapor explosions, but even with an isolated furnace there is still the need to seal off return air from the combustion chamber and check that weekly also, better be safe than sorry.