If they are planning a lot of short days of travel rather than a few long ones, they should look closely at their itinerary to see if it could be done by public transportation.
For the Bavarian portion, a Bayern-Ticket for five people is €53 per day (€10,60/person/day) and gives unlimited travel that day on regional trains and buses (www.rvo-bus.de) in Bavaria, including to Salzburg or Kufstein, Austria. For all of Austria, an Einfach-Raus-Ticket, for €47 per day (€9,40/person/day) gives unlimited travel that day on regional trains in Austria. In addition to trains in Austria, Post buses go many places.
Travel with these regional passes could cost no more than fuel for a car on days they travel and definitely less than the daily cost of car rental on days they don't travel.
I've traveled extensively in Bavaria using mostly Bayern-Tickets (sometimes just single trip tickets for shorter day); trains and buses have always gone to everywhere I wanted to go, and 80% of my travel has been to towns of less than 20K population.
You can go between Salzburg and Hallstatt using rail and just the Einfach-Raus-Ticket, or like I did, using Post buses through the Salzkemmergut.