I do not think it is possible to determine what is a rare car. Sure, the cars in you're example are, but what about limited editions or special trim levels of more common models, they can be very rare aswell.

But to know this you have to know very much about the specific make and model, only experts can decide in my opinion....

But I like you're idea, although I think we have to set up strict guidelines to determine what rare is.

I know one that is rare and that I learned about through this site: http://www.imcdb.org/vehicles_make-Rumpler.html