some sort of eurail pass or buy as we go?

Are you going from Vienna to Salzburg in a day with a stopover in Mauthausen, or spending the night in Mauthausen. Regardless, 3 or 4 days of travel hardly justifies a rail pass. Salzburg-Rothenburg: If you can commit and purchase in advance from the Bahn website, you can get express train tickets for as low as €49 for both of you. Express trains would take 4½-5 hours. You can also get a regional pass (Bayern-Ticket) for €28 (up to 5 P) and do the trip in 5½ hours, no advance purchase necessary. The trip back to Munich is probably best by regional trains anyway, a €28 Bayern-Ticket.

I agree with Lee. If you have an itinerary then it is easier and a lot cheaper to buy your tickets in advance. As-you-go can be quite costly, and the Bayern-ticket is a good deal.

A full fare 2nd class ticket from Vienna through to Salzburg is €95 for two. Spitting it into Vienna to Mauthausen and Mauthausen to Salzburg would be little more, maybe the same. So, put that together with two Bayern-Tickets and you have €151, $205 at today's exch rate. A 2nd class Germany-Austria 5 day Saver pass for 2 is $600. Any question? "it is easier and a lot cheaper to buy your tickets in advance." Usually, but not in this case. The only leg left is Vienna to Salzburg. Austrian Rail does not have discounted, advance purchase online fares for trips entirely inside Austria, so you are going to have to pay full fare for that leg. The only advantage to advance purchase would be to have reservations in advance.

Thank you both for the info.

Actually, if you leave Vienna early and don't spend too much time at Mauthausen, it would be possible to do the entire trip, Vienna to Mausthausen to Salzburg in one day with regional trains and use the Einfach Raus ticket, €28 for that part, too.