"The 600 was so heavy that the largest engine of Mercedes at that time, the 6-cylinder 300, was inadequate. Instead a new engine with more than twice the capacity was specially developed to move the vehicle and its hydraulically powered amenities, the massive 6.3L V8 "M100" engine with single overhead camshafts, and Bosch mechanical fuel injection" [Wikipedia].
Nothing needs to be done to it. Owner has painstakingly restored the car, like ordering two rolls of cream leather from Mercedes Germany to reupholster the seats and sending the interior wood fittings refurbished in Australia. The only thing that needs to be done is to rechrome a few exterior parts.
The car is so over-engineered that its not funny. The hydraulics (doors, boot), the suspension, the truly amazing seats, the way it drives - I can understand why it was the Maybach of its day - hence all those famous owners that ranged from royalty, to movie stars, all the way down to petty dictators (and even Jeremy Clarkson hahaha)! Like it was back then, its not a cheap car own - the hubcaps alone cost 3,000 euros to replace!