Buy a Bayern-Ticket at EUR 25 from any automat (less queue in the arrivals hall of Terminal 2 than in the Munich Airport Center where the station facilities are). Take the first available S8 to München Ostbahnhof (at xx.04, xx.24, xx44) arriving 31 minutes later. The xx.24 have the best connection to the Meridian trains to Salzburg.

No real advantage of buying EC/RJ tickets(EUR 27 if bought in advance, thus cannot be changed or EUR 39 if flexible)