I had a mid 80S Caddy that needed a new engine at 12 years old and only 60,000 miles. That's pretty crappy and expensive. I have an 86 420SEL that is one year older and has close to 200,000 miles now and has never had a valve job. The original suspension is starting to feel like it is on its last legs. I will probably have to replace major components and bushings over the next year. I expect the new parts will last another 12-15 years. I expect to get a valve job and replace the timing chain next year. Other than that there are no problems. I expect this car to last at least as long as it has already, maybe more. These cars are easy to maintain (can get a little pricey sometimes). You have a great car, its longevity will only be limited by lack or proper care.
1995 E420 - retired
1986 420SEL - retired