The only thing that "goes wrong with them" is that people skimp on the maintenance of that V12 engine.
Just to amplify my point about "one owner." The guy who bought this paid more than £150k (sterling) in 2003 - New Zealand car import taxes are horrendous.
As the car stayed in the hands of one owner, it has "probably" been well stored and maintained, which would include renewal of coil packs, maintenance of suspension, and all the usual annual maintenance points. But all of that is easy to verify. It's when cars go multiple owner that later owners start scrimping on the maintenance.
To apply a bit of context in terms of UK prices, a normal multi-owner 2003 S600L would still struggle to achieve £7k and would take an age to sell. (Hadleigh have been trying to sell a similar mileage S600L for £10k for THREE YEARS now)
2003/03 MERCEDES BENZ S600L 5.5 V12 500 BHP, 57k MILES | eBay