Well.......that's a deep subject

Sorry, couldn't resist. There are so many parameters and variables to consider when trying to decide if a W123 is a "smart" buy.
The list just goes on and on and on........ How are the mechanicals? How's the body (rust)? How does interior look and feel? Is it mostly original? Does everything work as it's supposed to? As you mentioned, no way to know actual mileage. So, that's a pretty big concern IMO. How much are you gonna hafta spend on it? If it's really cheap, then you can ease up on some of the previously mentioned details. Is the asking price really high? Then it better be worth it in fit, finish, operation, appearance, etc. Does the engine start quickly and run smoothly? How does it handle on a test drive......tight and smooth or loose and sloppy? Manual trans is def the best (manual windows, too). But, not a deal breaker if everything is working and functioning properly.
It really comes down to how much do you want to spend? How much work are you comfortable doing yourself or having done ($$$)? Used car prices are just insane, right now. I recently heard of a W123 selling for $20K! No doubt, top of the mark, excellent example. Worth it? Up to the buyer, I guess. If you could post a general condition, what's good and not good on the car and the asking price, the forum might be able to get you a little closer to your decision. Good luck!