Rent a small car. I often see Americans who cannot manouever their larger Mercedes or BMW out of the rental park house at MUC. Lanes are narrow, especially in towns, spaces in garages are not as wide as in America, and generally there is less space for street parking (and some creative parking as a result)
Hertz at MUC often runs out of automatics, requiring 'upgrades' to the vehicles most of us don't want in Germany ie SUVs, mini vans, etc.
Diesel will generally be more cost effective than a gas vehicle. There is a long thread here on how to drive on the Autobahn. It isn't what many Americans imagine it to be. The A8 from MUC to Salzburg can be one of the most dangerous and also one of the most blocked in Germany. You will get to know 'Stau', and it isn't a place you will want to be. Check for holiday periods; not just Bavarian ones, but northern ones as well as the A8 is the main route for travel from northern to southern Europe to holiday destinations.
Don't use the horn except in cases of danger. Gratuitous tooting as practiced in America and Asia just isn't done.
I would ignore the GPS recommendation. (Especially to purchase one here?!!) They are pretty much standard in German rental cars, and the American one with German/Austrian maps can be rather useless. If you absolutely want to guarantee one in the rental, you can book it at time of rental, but they are standard for pretty much every VW, Mercedes, BMW rental. Garmin never did manage to figure out the main park house in Salzburg is inside a mountain, and thus directs one to places where no vehicle can drive.