The twin turbo V8 M278 engine is sweet and one of the best Mercedes non-diesel motors ever built. It's bullet proof. They don't build them like this anymore. To address the op's questions directly, although the 400 may be a good car, I can't see it matching the performance of the 550.
The ML 500 isn’t sitting on top of the pile, though, because there’s a storming ML 63 AMG version, with a twin-turbo, 5.5L V8 that will dash from rest to 100kph in a sports-car-like 5.3 seconds. It’s a lot more expensive than the ML 500, though, and this car takes just 0.3 seconds longer to perform the same trick.ML 350 ML 350 SUV Package Includes. Price starting at. $51,725. Vehicle. Mercedes-Benz M-class Specs for Other Model Years. 2014 Mercedes-Benz ML350 / ML550; 2013 Mercedes-Benz ML350 / ML550; ML 400 4MATIC Sport Utility 4D. $64,795. $14,888. ML 63 AMG 4MATIC Sport Utility 4D. $100,115. $24,233. For reference, the 2015 Mercedes-Benz M-Class originally had a starting sticker price of . 138 186 1 338 11 355 109 248 344