For what it's worth...I have a 20x22 attached, un-insulated garage (except for the 2 garage doors, they are insulated) with an exit door, 2 windows and dry-walled ceiling. I needed to heat it for a party as the house just didn't have enough space for everyone.
I used an upright kerosene heater (23k BTU) and let me tell you, that thing threw the HEAT! When it was about 20 outside, it got up to about 60 degrees in about 2 hours. The actual day of the party, it was 40 degrees out and it got too hot in the garage! I had to turn it off before the party even started and then I used a 240v (4000w) electric heater to maintain the temperature. It worked great but when it was 20 degrees out, that electric heater would run ALOT and it was never as warm as the kerosene heater. Big difference.
Kerosene heaters are so common, it's likely that you someone you know already has one and they will let you borrow it. If not, you can probably find one slightly used for less than $100.
Burn the clear (expensive) kerosene in it and you barely smell it. The only thing I don't like is that it's either on or off, no in between. And when you light it or shut if off, you will likely see a puff of smoke and smell a strong kerosene odor for a bit, especially when the wick is dirty. I get around this by lighting it and shutting it off outside. It can be moved while it's on but just be careful.
As for moisture, I wouldn't worry about the added moisture from the kerosene heater in an un-insulated garage. If you ever do insulate the garage, then I would definitely consider a portable dehumidifier if you'll be using the heater on a daily basis.
...unless you insulate!
