Congratulations, Joyce! The Singer 206 is a fantastic machine (I've got several), and you've got one of the earliest models, what with the spoked handwheel and the old-school bobbin winder. That is rare indeed. The ones I have are electrified, but yours is a natural-born treadle. Just fantastic.
Please do make sure you stick with the 206x13 needles, they can easily be ordered online, at Sew-Classic for instance. As you've mentioned, a 15x1 needle can damage the bobbin case.
What's pretty amazing is that your Wittenberge machine, with the manual in German and all, was clearly brought over by the original owner on a ship! I can't imagine how they packed it back then, before bubble wrap and such.
And yes, this was Singer's first domestic zigzag. They previously had zigzag capability in some of their industrial machines, but the 206, which I believe was first released in the 1920s or '30s, finally brought this into people's homes.
The 206s I have are all made from aluminum; I'd read that there were cast-iron 206s, which I assume yours is as it was one of the first. When you tip it back on the hinges to access the bobbin, is it extremely heavy?