Given the information provided in the first post, I'd recommend the M5, which is a big departure from all the other M bodies, and, at the same time, preserves their essence. However, since you have a bunch of collapsibles... just don't use them with the M5.
With regards to the legends about serial #... the camera was issued in 1971. You can be assured that most of the bodies that had problems and were under 134xxxx have been repaired already. There's a similar story about M4-2 bodies (and I forgot the doomed SN), and, again, given the time these bodies have been in circulation, your chances of getting a lemon are relatively low.
If the store offers an M5 body with a working meter and a good warranty, I'd say go for it and don't look back. Believe me, later on, you won't look back. Period.