I'm late to this party but I've been using a ventless propane wall-mounted heater in my SE Michigan (cold climate) 2-car attached garage for over 10 years. It's approximately the same size as the one the O.P. eventually decided on. Mine being propane, I run it off a BBQ gas bottle which is located outside the garage; the fuel is piped in through the garage wall using copper tubing.
Some thoughts about this for those who may see this thread in the future:
First, the heater definitely does put out moisture. It condenses mainly on the inside of the uninsulated steel garage door; I have not noticed it condensing elsewhere. The amount of moisture is not extremely high in my opinion, and I have certainly had no problems (rust, corrosion, etc.) from the moisture. But it exists, and it is not a myth.
Second, somebody mentioned that any aerosol you use in the garage goes through the heater and comes out smelling worse. This is true. I would definitely not consider doing any significant amount of spray painting, or using beyond a small amount of spray solvents such as carburetor cleaners. They do find their way to the heater, they do burn, and they do smell terrible. As I do not do stuff with aerosols frequently, this is not a big problem for me, but it is worth mentioning.
Another thing in a similar vein: sweeping the floor raises dust, which also finds its way into the heater. You will see the flames go from blue to yellow, and not only will things begin to smell, but the heat output goes way down. Don't sweep the floor with one of these on. If you must sweep, turn off the heater, open the door, sweep, allow the air to clear, then close the door and relight the heater. Or, turn off the heater before the end of the garage session, then sweep. I have not tried using sweeping compound to lower the amount of dust produced, but that might help too.
If you have swept the floor with the heater lit, you will sometimes find the heater is less efficient afterward, and produces strong odors. The cure is to disassemble the heater and clean out the burner. Compressed air may not be enough; I have had to swab the inside of the tube-shaped burner with paper towel to get all the dirt out. See the above note about not sweeping with this type of heater lit.
A word about performance. The ceiling in my 2.5 car garage is approximately 8 feet. Most of the heat goes right up to the ceiling at first, then slowly filters down. I have been looking for an acceptable ceiling fan to mitigate this, but my overhead space is complicated, to put it simply, so it's not as easy as it might be for some people to find a unit that will work. Even so, here in MI we have an average daytime temperature in January around 25F; the "natural" temp of the garage under those conditions (probably due to heat losses from the house) is about 36-40F. Running the heater for a half hour will produce indoor temps around 55F; running it for several hours will eventually heat the garage to over 72F. I have done upper engine rebuilds in my garage in January and been warm enough to work in a t-shirt and jeans. 30K BTU in a 2 car garage will do the trick just fine, even with no ceiling fans, but it does take awhile.
I am out in my garage no more frequently than the O.P. I might use the heat for several hours one time a week, if that. For this limited use, BBQ propane bottles are of adequate capacity and reasonably economical; I might have to refill 3-4 times per winter. (Actually, I just ran it out last night and need to refill it for an average price of about $24.) With the local Home Depot operating an automated self-serve BBQ bottle exchange 24x7x365, the convenience is definitely there.
I am keeping my eyes open for a good used potbelly stove, which I think would be more economical for longer periods of use. Here in the north woods, firewood is cheap, often free, and very plentiful. Many people heat their garages here with wood. I'm surprised this wasn't offered as an option earlier in the thread. Additionally, if you have a modest amount of waste oil available, you can often burn it effectively in a potbelly woodstove; many people do so. Something to consider for those who have some waste oil available, but not enough for one of those big waste oil heaters.
Hope this helps someone.
--Geoff