The 6.9's M100 engine is repeatedly described as bulletproof, stout and never stressed, with owners reporting it can run enormous mileage without invasive work. The main acknowledged wear item is valve-guide wear on high-mileage cars, and commenters argue a strong compression test makes opening up the engine unnecessary, pushing back on fears of a prohibitively expensive rebuild.
The M100 6.9L V8: overbuilt and long-lived
Mercedes-Benz 450SEL 6.9 · mechanical