I didn't see it mentioned yet but the earlier M3s had a film advance that required two strokes to advance a frame (because the Leica folks thought a single stroke would have enough torque to rip the film). Later serial numbers have a single stroke advance. The point is that the double stroke advance cannot be repaired except by replacing it with the single stroke mechanism-quite a bit more expensive. There is also some legend that seven digit serial numbers (1 000 000 +) are "the best" because...well, I don't know why. Maybe it takes the Germans a long time to learn how to properly build something they created. . Seven digits command a premium. My 979 xxx M3 has been fabulous. You might find the M3's viewfinder limiting as regards 35mm WAs but Leica made a quarter of a million of them so you'll run into them a lot as you search around. You're across that ocean but you might search/email Youxin Ye, a well known Leica repair/sales guy. He always has some cameras around and seems to be a pretty straight shooter. He CLA'ed my M3 before I purchased it (from another party).

Good luck!