For four people, a Bayern-Ticket is only 35€. It is an all day ticket for unlimited rides in the Bavaria fare zone. You can only use it on regional trains, any time weekends, after 9 AM on workdays. Regional trains take about 2 hours from Salzburg to Munich, about half an hour longer than express trains. There are no seat reservations for regional trains. During popular times they can be quite crowded, and you could have to stand, but if there is possibly room, they'll let you on. Regional trains from Salzburg to Munich start in Salzburg, so if you get there early, you would have a good chance of getting a seat.
Bayern-Tickets don't sell out; they are always available at the same price. You can purchase them anytime, right up to train time.
With advance purchase, you can get Saving Fare tickets for as low as 49€ for four adults. That's 19€ for the first adult, 10€ for each additional person over 14 yrs of age. So, if your group includes 2 children (14 or under), they travel for free (if included in the ticketing), and the ticket is only 29€. With express trains, you can reserve seats (9€) for a family reservation.
Savings Fare tickets can be a lot less expensive than full fare tickets so it is worth getting them if you want to travel on express trains (ICE, IC, EC). However, they are train specific; they can be returned for a penalty up to the day before travel, but not as of the day of travel.
The trip from Munich to Frankfurt takes a quite a bit longer with regional trains: you probably want to get advance tickets for an express train with reservations. Most ICEs, except for those that go through Nürnberg, stop in Pasing on their way out of Munich. Dachau Bahnhof to Pasing is a 22 min S-Bahn trip with a change in Laim.