According to :

I only want to use this occasionally when it I have
to fix something or work on my car.

If you're only going to use it occasionally, a construction
cube heater (4800W 240V fan heater) has none of the drawbacks
of wood/gas/propane (moisture, possibilities of CO, venting etc),
convenient/safe (eg: leaving it running for a day or two),
and its extremely low installation cost will offset operations cost.

In my situation (garage in the great white north is well insulated,
and probably about the same size as yours), any other form of heating
would have a 20+ year payback compared to the two cube heaters I use
because of the installation/equipment costs of other systems.

Electric IR panels are another possibility - they're relatively
small (eg: 1000-1200W), but keep _you_ warm while working on a car,
and can be plugged into ordinary outlets.

Lee Valley carries one of these panels. $70 I think.

I've found cube heaters for about $40 new, and bought one at an
auction for about $15.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.