I don't think you will be that happy with an electric heater. Then again, you're the guy washing at 24 degrees
I would suggest a very small torpedo heater that runs on propane, or kero. For $100, your garage will be easily warmed and you won't have to worry about waiting. Depending on your ceilings, I bet it give you 40 degrees. I.E., if it's 10 outside, the heater will raise it to 50.
Shumax