I bought mine a year ago, and in spite of my love hate relationship with the car, it is still my dream car. My 99 S500 is my second Mercedes, the first being a 420sel, has enough power to move like a F16 in a dog fight with a Boeing 747. She is truly a contender for the greatest car of all time, certainly a top 3 finisher, and even at 14 years old she can still turn heads.
The way she handles, rides and the POWER. The fact that it makes enough noise that people stop and stare yet it can be quiet an subtle.
Maintenance and parts costs are outrageous yet, often validated...so I would say change nothing except fuel efficiency. I average about 13mpg and thats a combined city and highway number.