I'm guessing wood is out because you need something that will operate unattended. Have you considered waste oil, corn, or wood pellet heaters?
The type of stove you would choose depends on whether you can get a deal on the fuel....got any farmer relatives in the corn growing buz? Pellet and corn stoves have a little auger that pushes fuel into the burner when necessary. Up in my corner of the world, lots of people use pellet stoves for heat.
I've looked into waste oil heaters but they are a little more maintenance and I'm not sure they are allowable in an attached garage. I'm pretty sure they are ok for use in a detached workshop building. Waste oil heating would be ideal for a guy who scrapped out cars since they have to drain the oils anyway.
Hope this helps,
Jon.
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