Are you sure you really need a M6? Or maybe you're better served by other cheaper and newer options?
I have an M6 that I bought used. It is very very smooth and I bought it because of this. There were several samples I tried before, the variations in feel could be quite huge. Try a few first physically before you buy one. Also, external cosmetics tell nothing about the internal mechanism. The one that my friend has initially developed problems with the film advance mechanism, had to get it fixed with a couple of hundreds. Turn out that some parts were badly bent from a poor repair job. His looks very mint too.
The lens in the camera for the light meter may also have fungus that is difficult to clean.
Physical view of the item is important. Sometimes gear that seemingly would bring us to higher levels of happiness may give us more headache instead.
Not sure that's why i am borrowing from you to try. Bet you didn't know who i am from my nick
