I've heard of Zundapp, but I've only seen their scooters with that big funky front fender. I got a cursory education from Wikipedia on the brand. The flat twin and the driven sidecar makes me think Ural, but this is obviously not a BMW ripoff. Is there any connection there?
I could Google some more, but could someone give us some more history for the good of everyone reading? Were they ever sold in the States?
BTW, the bike looks very complete. Does it run? I'll be very interested to see the progress.