Mike,
Have a look at the Bahn.de website to see all the train solutions on those routes - https://www.bahn.com/en/view/index.shtml .
The trip from Munich to Salzburg is relatively short with direct trains taking 1H:45M. There appear to be Savings Fares currently listed at €25 PP.
For the trip from Salzburg to Vienna, there isn't much difference in travel time between the RailJet and Westbahn. Choose whichever one fits your schedule and has the best prices at the time. You can buy Westbahn tickets on board, but if you're planning to use a credit card, I believe they only accept Chip & PIN cards. Check their website for details.