Every system in these cars has it's own life expectancy. And they vary from one system to the other. Engine is the least of the problems, and has the longest life expectancy.
For me,....I would want the service records from the day one. No service history - no deal. With 180k on the clock, I know some systems were repaired/replaced/upgraded, or the car would not be driveable with these miles on the odometer. Question is,.....what was replaced, and when. How old are the Airmatic components, tranny, and electronics? Normal wear items are not a problem. Brake system components are cheap, and easy to replace. Rebuilt transmissions are available, and the existing one can be rebuilt/swapped. Airmatic can be yanked out, and conventional, coiled shocks installed.
So it all comes down to math. With these many miles, you are likely the last owner of this car. Selling it in few years is not an option, because the value will not be there. It will be more valuable as a luxurious transportation to you, than the couple of thousands you could get for it.
So do the math. Transportation costs money (like anything else!). If you "invest" "X amount" + repairs (the immediate ones + future), divide that with number of years you will be driving it, and you will have the appx. cost per year/month. If that number is ok with you,.....go for it!
How much is your transportation supposed to cost? Only YOU can answer that question. Is it 5K per year, or less, or more?
Sometimes not having a payment (like on the lease, or purchase of a new one), does not mean "winning the game".
The above said is true only in the "average person's" mind. But there is that "afficionado's" mind, where some object (in this case a Mercedes), becomes the passion, and a hobby of some sort. In this case the math goes out of the window! Most people on this forum (and many others like this one!), belong into this "unreasonable" group. They take pride (and a form of challenge) to keep these cars going without taking them to the dealer, learning EVERYTHING about all the systems, how they work, where to get quality parts on the cheap, and how to DIY.
So,....it's time for some introspective thinking. Which group do you belong to? What is your finanncial situation? What are you buying here (transportation, passion, status, or something else)?
Would I buy a car with that many miles on? Sure,....depending on the price, and the car's history. Should you? Only you can answer that.