I have always used AutoEurope or Kemwell for my rental cars. You can check their rates online and then call them to reserve your car. If you find a rate cheaper somewhere else, they will match it. Make sure you check the "terms and conditions" for Hertz, Avis, Europcar, etc. There are differences between the companies and added on charges. Example would be that with Europcar, an additional driver is free of charge if that person is a spouse or family member. Personally, I avoid Avis and Hertz since on previous rentals they try to charge me more either at the beginning or end of the rental.
Additional suggestions would be: request a diesel for better gas mileage; take pictures of any damage and take video of the car when turning the car in; note any scratches/dings at pick up
Even though a gold credit card can provide insurance for your rental car in Europe, I also purchase inexpensive travel insurance from Travel Guard or Travel Insured. In addition to accident coverage, this insurance covers "loss of use", tires (no spare tires are in rental car's trunks), side mirrors, undercarriage, etc.
GPS. I take my own TomTom GPS with European maps which is a lifesaver. Renting a GPS from the rental company is more expensive than buying your own GPS. Many European cars come with the GPS installed so I would not pay for a GPS.
Make sure you take copies of your contracts. I have had Avis try to do a switch on charges on me.
For Salzburg, I stay on the outskirts, park the car and take the bus into town. Haus Ballwein is a beautiful B&B with a great buffet breakfast and peaceful surroundings. Just a suggestion.
I have rented a car at the Hauptbahnhof in Munich and it was fine driving out of the parking garage and finding the autobahn. It'll be that much easier with a GPS.
Enjoy your trip.