I guess I am just lucky, living right, or something. My '88 300E has 182,000 miles and I have yet to replace any exhaust components, A/C components or motor mounts.
I am, however, nervous about the a/c. These are known for their problems. I cross my fingers every time I turn it on. But, if it breaks, I will fix it.
I believe that on the '90 model they had the valve guide problem corrected.
I have had mine for quite some time, but did not start driving it daily until about nine months ago. But, even with a head corrosion problem that I have experienced, (due to lack of keeping the antifreeze changed by the previous owner and, I guess, me too) I haven't exceeded $1K per year in repair costs.
These are fabulous driving, very reliable cars. My suggestion is to change oil often, stay up with the preventive maintenance and enjoy it. If something goes wrong, fix it.
It is totally beyond me how people can predict that a part or system will fail at a particular mileage. I guess they're crystal ball is MUCH clearer than mine.
Drive it and enjoy it,
Change oil hot and change oil often,