Apparently the hardcore Amish don't allow any instrument besides the harmonica (but only first position playing :).
However, the Amish came from many parts of central Europe. So there should be more styles than the umpa as influences. Also, since the emmigration there hasve been, though limited, also some influences from the surrounding communities.
However, this music is, if I had to guess, more 'german' than the cliche music that you find in germany as 'traditional' today.
As I listen to it again... well, this sounds more like oldschool french music than german...
Also the drumming reminds me more of the celtic tradition (i.e. breton, irish).
However, the point of my previous post, is that the umpah music is obviously a cliche.. Plus I'm not sure what kind of music they really played 150-200 years ago because this has also changed a lot over here. And I'm sure that there were big differences from area to area... So when it comes down to looking for original oldschool harmonica music the amish is probably a pretty good place to start.
Well, I did a little search for akkordeon, cos I remember finding a lot of oldschool stuff there. Maybe you could try copying their melodies.
The thing is... harmonica music being based on the tonica and dominant chords is actually conform to a LOT of music. I'm sure that once it was introduced it really spread quite rapidly and was intruduced to various different styles. So looking for traditional german music is surely not wrong however I' say that many kinds of European music shoudl be fine - just work those tongue-slaps..
One other very popular style that comes to mind is the german sailor songs (Seemannslieder):
