The one in your first link would not cut it, especially in a garage with any drafts. I would look for another Reznor, or have yours serviced, or pick up a regular residential forced air gas furnace. There are a lot of people stepping up to high efficiency furnaces in their homes and discarding there still working old units. That is where I got mine, 80,000btu heats up my 720sq.ft. garage quickly and for the cheap. If you do go with a house furnace make sure to keep the return air off the ground at least 12", maybe more check you local codes, to prevent gas fumes from entering it, and vent it to the outside with double wall flue pipe.