I bought a vent free 30,000 BTU natural gas, wall mounted heater for our garage. It also has a 110v blower on it. 7 years ago, it was $140 dollars. I installed it myself. You can get them either natural gas or propane. Your local furnace shop will make a flexible gas connection pipe to the length you need. If you put a vented 'Modine' type heater in, there are clearance requirements..........like in Michigan, you have to be able to walk under it. Ceiling height may be an issue. Our garage is under the living space, insulated and about 900 sq. ft. This heater works well enough when I need to work out there, but I don't run it except for work. If you live where roads are salted heavily like we do, keeping the garage heated causes salt to do it's nasty deed faster on your other vehicles. Our garage temp is typically between 30&40 degrees with no heat. I can raise it to between 50&60 with the outside temp no lower than 20 or so. I do have another natural gas outlet I put in the back of the garage where I could throw in another unit, but don't feel it necessary.
Steve