I’ve always had an oil filled radiator in mine. That, or a ceramic heater are the two cheapest to run. Fan heaters and convection heaters are more expensive to run.
If you have condensation in there, I’m sure the steel parts of the car will rust. I had a chrome wrench in my garage before I started heating it, which turned rusty, just from condensation.
I've just ordered a ceramic heater from amazon, £23 includes free delivery. It’s very small and with a built in plug. I just want to keep the chill out of the air in my garage. A colleague has one and it costs about 1.5p per hour to run.
Hope this helps.
Qooker Fan Heater, Ceramic Electric Space Heater With 400W/650W Power Setting, 2 Seconds Heat-up, Tip-over and Over-heat Protection, Portable Heater for Home, Office, Indoor Use - Black https://www.amazon.co.uk/dp/B07MTX6P4P/ref=cm_sw_r_cp_api_i_il1wCbPQV0S4P