Garage Heat

No one has mentioned using a Hot water heater for radiant heat. If your concrete work is already done you can run radiators in the garage instead.
The are efficient and can be NG or Propane or electric.

I use one running off my Pellet stove in my house and then pump the heated water to where I need it. Soon I will have a shed just for it and pump to house and garage.

Using radiant you have no concerns about exlposions while painting or venting it unless it is a NG or Prop.