Scarce, yes. But a quick search shows that you can choose from 10 Z1s on one site if you want to buy one. Rare is when you have to look for 6 months to find even one for sale. A 507 is truly rare! So is an M1, which I'd much rather own than the rather odd Z1 - but they do have similar noses!
A chap I knew owned what was probably exceptionally rare, a 1950s BMW sedan with small V8 (502, I think). Although they did make a fair number of them, they weren't valued that highly and many were allowed to rust away, so I expect that there are few still in good shape today.