The naturally aspirated 5.4L M113 in the C55 is repeatedly called the most reliable engine AMG ever built, with owners citing examples well past 170k, 200k, and even 340k miles on original motors with no oil consumption. The consensus is that simple, self-serviceable maintenance keeps them running almost indefinitely.
M113 V8 (C55) reliability and high-mileage longevity
Mercedes-Benz C32/C55 AMG (W203) · mechanical