If you want the flexibility, buy the Bayern Ticket Single at €21 for 1 person (assuming you do not start your journey before 9am Mon-Fri). You have a train (with one transfer) approximately every hour. These are the connections showing up at €21 on your search (there are no €21 Sparpreis fares).
Although you can only travel on local trains (so no RJ nor EC), the overall journey is only about 15 minutes longer than on the fast trains, because you can change at Munich Ost (take the S8 from the airport, not the S1), and do not have to go to the Hauptbahnhof.