One of the Munich days I am planning on doing a Dachau tour, so it
won't be spent in the city.
Dachau is a separate municipality, but it is just outside the Munich city limits. I would hardly say that your day won't be spent in the city. You should be able to get back in the city center (Hbf, Marienplatz) by mid-afternoon.
BTW, I hope you mean you are touring Dachau that day, not taking a cost added, third party tour. It's really easy to get to the Memorial on your own, and it costs a lot less than the tours. I took the 2½ hour tour given by the Memorial (4,-€), and I thought that tour was excellent. Our guide told us more about Dachau than I could ever remember.
You buy an MVV M,1 Day-ticket (10,50€ single, 19,20€ 2-5 people). That ticket covers all of your transportation (S/U-bahn, trams, or buses) on the Munich Metro all day long, including round trip by S-Bahn and bus to the Memorial. Leave from any S-Bahn station ( Ostbahnhof, Rosenheimerplatz, Isartor, Marienplatz, Stachus, or the Hbf) on the trunk line through Munich. You don't have to go to some meeting point and wait.
Take the S2 to Dachau Bahnhof, about 25-30 minutes depending on where you board it. The S2 runs four times an hour, 10 or 20 minutes apart. At Dachau Bahnhof you take bus 726 (it's well marked). It runs every 10 minutes and takes 7 minutes to get to the Memorial.
Admission to the Memorial is free. The excellent tour offered by the Memorial cost 4€ and takes about 2½ hours, or you can rent an audio guide and do it at your own pace. There is also a free half hour film about the Memorial given in English multiple times a day. Check the website for times.
If you get to the Memorial early enough, you'll get back into downtown Munich in time to do some sightseeing and have dinner. And that day-ticket is valid for any other transit in Munich the rest of the day.