In Austria you'll need to have a special sticker on your windshield called "vignette", that is the equivalent of paying a toll. Actually, you should purchase it before crossing the border. Any gas station along autobahn A8 (Munich-Salzburg) should have it.
If your vacation is going to be in the Winter, make sure you get winter tires.
Also, your dream of driving without speed limits on German autobahns (Autobahnen) can, and certainly will, be shattered by lots of construction zones, traffic jams and areas that do have speed limits.
Another thing - please anybody correct me if I am wrong and/or add more precise info: if you are driving over 130km/h and have an accident, your insurance may deny coverage.