I second the comments of the previous respondent. Pay at the pump is very uncommon in Europe, but I never remember having to pay in advance. Just fill up and go inside to pay (just know your pump number). While pay at the pump is uncommon diesel is very common - in fact, in many locations, you'll find a couple different grades of diesel. Fortunately, regular diesel is normally a bit cheaper than unleaded (or at least is was in Belgium over the last year). You'll also see a fair number of Compressed Natural Gas pumps in Germany - these pumps look much different than normal gas pumps and they are normally separated from the unleaded/diesel pumps.

Have fun!