I took the tour provided by the Memorial 6 years ago, when it cost 2,50€. I've only been there once, and after taken the guided tour I had no interest in the audia guide, so I have no opinion of it. However, I have used audio guides on several occasions and I found them to be very good. If you can't get there in time to register for one of the guided tours, at 11:00 and 13:00 most days, I wouldn't hesitate to do the audio guide, but I would prefer the tour with a guide because you can ask questions.
I took the 1 PM tour and after it was over, there was only one documentary film left in the day, in German. If you plan to take the 1 PM tour and don't understand German, you need to do the film first (11:30 or 12:30). If you take the 11:00 tour, you can watch the 2 PM film or, since you probably won't have eaten lunch, grab a sandwich at the cafeteria and catch to 3 PM film.
If there are two to five of you, buy a Muenchen XXL (inner 2 zones) Gruppen Tageskarte (day ticket for 2 - 5 people) for 14,80€ (only about 3€ more than a day ticket for just the inner zone, which you would probably want anyway). This will cover you travel to the Dachau S-Bahnstation on the S2 (Richtung, direction, Petershausen) as we;; as the #726 bus to the Memorial, which you will catch in front of the Dachau station. The bus stop will be well marked. It's easy to get to on your own; no good reason to buy an overpriced, 3rd party tour package.
You can find complete information here.