Don't know what is the primary heat for the house. Oil, gas, electric, hot air, hydronic? Maybe a zone / loop can be added.
Not too comfortable with unvented combustion type heaters indoors.
Agreed. Even in an incomplete structure partially open to the outdoors the fumes can build up. I would not use one in a closed garage.
I'd probably go with two smaller strategically placed infrared electric heaters to spread the heat more evenly than one large one. You also might be able to use a normal 115v outlet that way.