I've got a 1983 Fuji S12-S 18 speed mixte - I'm of the opinion that this bike is representative of about the top quality level one generally finds in vintage, mass produced mixtes - cro moly frame, forged dropouts w/integrated derailleur hangar, componentry to match. Pretty cool bike that nobody would say is low quality.

However, most of the mixtes I see are hi ten steel w/stamped dropouts. Nothing the matter with these bikes at all, especially for neighborhood runabout, etc. But if you see one w/the better features, and they definitely are out there, hop on it. I think most of the manufacturers listed in this thread had at least one mixte w/cromoly and forged dropouts.

Your average seller is pretty clueless about these things, fwiw, and is unlikely to assign a premium to it, so you may as well hold out for it.