Shop/Garage Heater suggestions please

10KW is not a lot of heat, especially for a leaky building without much insulation? You are talking "about" 3400 BTU per KW, so 10KW is about 34000. Without knowing how cold it gets, how fast you want it to warm up, the condition of the building, etc, that might not be "much" but of course it WILL take the chill off

Something that is/ was popular around here was to find an old electric furnace from a heating changeout/ upgrade. There's not much to them, heating elements, a few controls, and a blower. The "standard" common heating element is either 2500W or 5000W or about 20A at 240V If you got one with at least two elements, that would do it, or you could buy a larger one and leave more than two disconnected

The other thing is, get ahold of your utility and compare the cost of electric to "what else" you might consider, LPG? Oil? Many places with expensive electric, LPG can beat that cost some.