I have a 1995 Mercede-Benz E420 in Vancouver, BC. I'm not very technical, so I can't really tell you about mechanical issues, but I can tell you about my general experience with the vehicle.
My car has around 30,000km on it, since your Canadian too, I'm leave it as metric instead of converting to miles. The car was bought new from the dealership and I've pretty much stayed with the maintenance schedule as indicated by Mercedes, although my mileage isn't very much, so I've been basing the maintenance on the recommended months.
I think the warranty expired on the vehicle (3 or 4 years???) and I remember general maintenance was around $300CAN, and more recently, I had a more major maintenance where I had my brake pads replaced, and that racked up to a wopping $900CAN. Between the maintenance periods, the only problem I've ever experienced was my engine light coming on. This happened twice actually. When I brought it into the dealership, they said it was a faulty O2 sensor and replaced it on both occasions without cost, because they said it shouldn't have happened since the car was brought in for regular maintenance, which I found to be a nice gesture. From looking at the bill, I think replacement of the O2 sensor would have cost me $330CAN (I had a peak at it the second time before one of the Mercedes mechanics tore it up sayin' that it wasn't right to charge me for the fix).
In terms of overheating, I've never experienced it. The weather stays mild mostly year round in Vancouver, so it hasn't been an issue. I did notice that the engine is quite hot, even after it has been sitting in the garage overnight. I guess this is the overheating issue people have been talking about, but it has never given me any problems, and I've monitored the temp gauge while driving.
Anyways, Whatever car you choose, I'm sure you'll enjoy it. Best of luck!
1995 Mercedes-Benz E420