Hi,
well the weather - you can be lucky in the last couple of years it was quite nice and warm into november. I wouldn't say there will be a lot of snow.

So driving will be no problem. Although if you want to go to Salzburg you can get a Bayernticket for the train which runs every hour and takes about 2hrs to get there. No need for a car at all. Of course if you want to drive around and explore the car will be better. There are some nice towns on the way to Innsbruck - especially if you drive towards Garmisch Partenkirchen and over the Mountain. Again, driving should not be a problem weatherwise.

Doing one day in Munich is a not much. Honestly if this is your first trip and as you have limited time i would skip Landshut and Regensburg. They are nice places but with the time limit i would not go. Landshut by car is about 45 Minutes to an Hour - Regensburg even further.

Take the S-Bahn (Suburbantrain) to Dachau, do the concentration camp, the castles in Munich, Olympicpark etc. you will see more than enough.

Hope that helps. If you need more info feel free to PM me )
Chris