I think that's a major concern on any camera that old, as well as lenses. I had to dismantle a Summarit to get rid of the flakes of black paint on the inside of the lens. Any number of things can impact whether or not the paint will flake off - heat and changes in moisture, even the possibility the painted surface wasn't quite clean enough when painted. All of those things are beyond your control, since the camera (or lens) has had a life of its own before you got it. So, if you examine the camera and see the rangefinder patch looks good and you don't notice any paint flaking (even just looking inside the camera with the lens off), there's not much more you can do. But anything can happen to it once you have it.
I'd assume Canadian M3s would have been made from Wetzlar-supplied parts, but assembled by the staff in Midland. You hear about poor material choices in the (early) M4-2 but you don't normally hear anyone say the staff was not trained properly or careful enough in assembly. Any M3 made in Canada is probably just as reliable as a similar-vintage one made in Germany.