I would think if you get something that runs on lp without a fresh air intake for combustion, in a situation such as a garage you will be sucking in as much cold air as you will be heating. A garage with a cement floor will be really hard to somewhat warm up without being on all the time. Heat rises, the cement slab will stay cold if you just try and heat when you want to use.
Insulate the door best you can. Replace the trim wipe strips on the door if needed, and adjust the roller channels so the door seals up. I'm afraid if you want to use it year round, even if it means 50-60 deg for winter shooting, you will be running a heater full time as it takes too long to heat up a concrete slab that's gotten cold. I have in floor heat in my basement, garage, and shop, and if I decide to work on my boat in the winter it takes hours to start to bring the temperature up in the garage, but it get's really cold by me.
Something else to think about, Maybe there is a club there, or at least a place to shoot that has a yearly membership fee? Cost wise might be a whole lot cheaper than trying to heat an uninsulated garage in a rental place. If it was your own house, you would have many options of converting the garage into a pool room, but being a rental, makes it somewhat difficult.
Hope it works out as there is nothing better than a table at home.
I think you were near Fulda, I used to go there often. I was stationed on a radar site at Wasserkuppe Germany late 68 to mid 70.
The radar site has been torn down for quite some time, but looks like it is still a ski resort and possibly still a glider spot.
Absolutely loved it there, used to ski down to my apartment in the winter and grab the lift back up to work.