Most problems are age and mileage related - bearings, vacuum (rubber) hoses and diaphragms, etc. The engine and drivetrain are in perfect working order. I live in Florida, but the car is from Penn originally, and it's in great shape.
Some major items fixed:
Tie rod ends, misc. bushings - $500 - 270,000 miles.
A/C vacuum system and re-charge (major overhaul) - $1,200 - 261,000.
New head on the engine - $2,500 - 225,000 (in records).
Other than that, it's mostly been minor items ($50- $350) for comfort and convenience items.
Also, forget the dealers, who are ridiculously expensive. If you're in a larger town, there will be a Mercedes specialist who will do better work FAR cheaper. Mine charges $60/hr for labor, and lets me bring my own parts. WOW!
This car is amazing. We call it "the beast" because it takes abuse and keeps on ticking. 14 years old, 271,000 miles, and the IDEAL daily driver.
It's important to buy one that has been properly and regularly maintained, as an older S-Class that's been neglected will easily cost more to get back into perfect order than it cost to buy. I consider a car with accurate and complete maintenance records (mine is one) much more valuable than one without.
If you're looking for a car that will provide a lifetime of service without killing you in maintenance and repairs when it gets older, do yourself a favor and consider a 1986-1991 S-Class Mercedes Benz. The gas models will be more expensive to buy and maintain, and the diesels will last forever. I plan to will mine to my grandkids, and I'm only 30.