Your Garage Heating System

This depends on whether you do ANY painting or cleaning with flammable chemcals.

Even if not, it would be a VERY wise idea to use a unit type or other heater which puts the burner up high, AND ONE WHICH HAS a separate intake for combustion air, a so-called "sealed" combustion chamber.

But you didn't say what fuel, if any you want to use, or electric?

Be VERY aware of the dangers of combustion devices. Even ignoring the explosion hazard from fumes of painting, cleaning chemicals, etc, chemicals such as in the garage can be VERY corrosive and damaging to the combustion chambers/ burners of heating devices.

I used to maintain this stuff. Even such places as photo development/ print shops, beauty parlors, and laundries, the chemicals in the air in those environments are VERY hard on heat exchangers, both AC and heating.

So, once again, a SEALED burner with outside combustion air is very desirable.