A single trip ticket from the airport to the center of Munich would cost 11,20€ per person and would include transport by any means of the MVV (S-/U-Bahn, trams, buses) to get you to your final destination in the inner zone as long as are moving continually towards it. However, a Group Day Ticket for the entire MVV would only cost only 23,90€ and would be valid for up to five people all day long, on whatever means of transport you desire, even after you arrive at your hotel. You might want to use other mean of transport later that day in Munich so the Partner or Group Day Ticket would be your best deal.
Beyond that, just about the fastest route to Rothenburg and back uses only regional trains (Bayern-Ticket). You could go via Nürnberg and use the ICE, but that route would not save much, if any, time and would be much more expensive. Use the regional trains and the Bayern-Ticket.