Take a deep breath and enjoy the fruit of your labor. Driving in Germany is easy. The signs are pretty easy to figure out. If you have a Rick Steves Germany book, review the road signs in the driving section. I used GPS on my phone during my May trip; it worked well --better than the GPS supplied in my (Ford) rental car; the GPS in Volvos and German cars I have rented worked much better than that in the Ford.
A couple of words of advice for the Autobahn: do everything you can to make sure fast cars can go fast. That means, stay right except to pass -- no driving the speed limit in the middle lane like people do here in the the US -- stay right! If the car in front of you is slow but faster traffic is coming behind you, let the faster traffic pass before you pass the slow car. Because cars will be traveling a lot faster than what you may be used to seeing, make sure you look a lot father behind you in the rear-view mirror than you typically do.
You can get your Eagles Nest tickets from the ticket booth next to the parking area, but, if you are going to see it as independent travelers, I would suggest getting there early before the tour buses start arriving. Don't miss the underground bunkers that are part of the Documentation Center. If you want a bit more of a comprehensive experience of Obersalzberg, including Eagle's Nest, then consider the tour offered by Eagle's Nest Historical Tours. The company offers excellent private tours, too, though I don't know what their availability would be at this point.
I'm not sure where you are staying in Salzburg, but, from the old town, take a walk along the river away from the old town towards the new town. Maybe walk down to the bridge where trains pass overhead (10-15 minutes), and then turn around and walk back. The walk back provides very nice views of the old town and fortress. If you are really adventurous (and know enough German to be able to order food/drinks), walk all the way to the library (Stadt:Bibliothek Salzburg), check out its nifty architecture that is reminiscent of a dinosaur, grab a drink/snack at its Panoramabar that offers nice views of the surrounding mountains and the fortress in the distance, and then walk back to the old town (about 30 minutes in each direction).
Good luck and have fun!