I did almost this exact same drive this February. Munich>>St. Johann im pongau
Rented from Enterprise a 9 person van. We had 5 plus ski equipment. It would have been fine for 9 people though. Enterprise has a easy to find booth in the MUC airport.
Make a quick stop in Salzburg. Nice downtown area. Also- redbull has a nice FREE museum. Worth a 20 min stop for a stretch.
The drive was as really easy.