Hints on looking for old Peugeots:
If the rear stays are half-chromed and it's at the thrift store for five bucks, buy it immediately. Look also for "Inoxyadable" sticker on the seat tube.
The switchover date from french to English threading is early eighties.
Anything lugged, brazed and built with double butted tubing is worth buying. You can also check the bottom bracket, that's where Peugeots have their serial #s. Late seventies ones have the model number after the serial number, ie. PXN10E. Anything with a "P" in the model number is probably worth dropping the cash on.

French BB threading is 35X1. Most of the upper end peugeots had nearly indestructable stronglight BBs. In the event the BB is shot, the phil rings and shimano UN72 BBS work nicely. The headset can be replaced with English bottoms sometimes, just leave the french tops. Or you can replace the fork with a newer one, and switch the headset entirely over to English.