BAP didn't really get going until the early eighties. I think "Iradeuss" ("Gerade Aus")and "Krystallnaach" are from "vun Drimme noh Drusse", which is about the third album. I like the songs too though. I was lucky enough to have one of their early producers (Manes Werr) do some stuff for me back in the early eighties. As a surprise Christmas present, some of my students gave me a ticket to a BAP concert a couple of years ago - and it was a good gig too.
      Getting back to folk songs, I agree with Wolfgang that Die Gedanken Sind Frei - one of the best known German folk songs - is always a good choice. It has the added advantage for a ladies' choir of harmonising well.