It depends on what kind of owner it's had and where it's been parked.
Yes, Ontario winters are hard on any car, and you'll be hard pressed to find any 15 year old car in Ontario with zero rust on it.
That said, the E34 seems relatively rust resistant, and frequent washes help a lot. My car has very little rust on it, and I think the previous owners didn't do very much to make it that way.
If you feel good about the previous owner, the records, and the photos, I don't see any reason not to go ahead with it.
The car you're looking at has probably spent most of its life in a dry, warmish garage. I'm not a metalurgist, but I wonder if that's really any harder on a car than being parked outdoors in Vancouver, where there's no salt on the road but there is salt in the air.