So many questions:
Where are your? Florida, Alberta?
How many garage doors? What size?
How often will they be open?
How often are you in there, for what time period?
Are you in a city or way out in the country?
Dirt floor?
Connected to a house?
Business, storage?
I am partial to overhead radiant tube heaters, but if you dont have a gas line, access to propane, its out. Also if your ceiling is blocked by the loft, there may not be height for one.