I purchased one of the NG blue flame heaters from Lowe's about 7 years ago. It's rated at 30K btu's which is overkill for my 2 car but is great as it heats it to 70 QUICKLY which is nice after you are working outside and want to bring it up to temperature. They have these in propane as well. It also has a thermastat built in and I added a blower. Couple this with the ceiling fan I have and it circulates the heat quite nicely. I can work on and wash our cars in the garage all winter long and maintain temps from 60 - 70 without issue. It's quite nice getting into your car and it's the same temperature as your house
The only downside is with a flame heater it generates moisture so you will find some stuff starts to rust but nothing major. Like I said, I've been running this for over 7 years without issue. This is by far one of the best things I've done to my garage
