We will arrive at Munich Airport with no Euros in our possession and need to take the S-Bahn train to the hotel Isartor. Can I purchase tickets for the S-Bahn with a Visa credit card at the airport?
Yes, but you may need a PIN for your credit card to work in a ticket machine. No problem if you buy at the DB Travel Center at the airport.
You can also use the MVV app to search routes, buy tickets, etc: https://www.mvv-muenchen.de/en/journey-planer/mobile-services/mvv-app/index.html
I you are travelling by train and therefore will oder should have the DB Navigator: it sells MVV tickets too. No need to download an extra app.
Whether or not you have luggage, your way out from Terminal 2 will take you through the baggage claim room and there is an ATM there (Deutsche Bank, I believe). You can get euro there.