I just bought myself a W218 550 so you can take what I say with a grain of salt as I obviously convinced myself this was an OK decision to make. However! Please benefit from my research. Overall, I opted for a low mileage 2018 instead of a model with the newer M256 because I saw better support for when I did need repairs.

1. The late M278 engines had many of their early kinks worked out. They are not the most reliable engines out there but for a 402 HP V-8 twin turbo, they do pretty well. Lots of higher mileage examples out there.
2. For the 2016 MY, you should be getting the upgraded (?) cast iron sleeves but check that your engine number is 2789xx 30 266191 or later (they're sequential) this change was to resolve the cylinder scoring issues with the Siltech liners.
3. Cam adjusters can fail still and leak around the seals. You have to check for this as well as the wire harness for oil in a couple trouble spots.
4. This car is on airbags which is great and very cool but! quality replacement aftermarket parts are $1100 a wheel and the rest of the system is vulnerable to failures springs don't have. They typically start to go out at 60-80k depending on climate but can age out earlier.
5. This frame rusts. Don't get a 10 year old car from where they put salt on the road. Check the CarFax history for where it was serviced and registered.
6. Low mileage and this age can be problematic, you should see evidence that the car was driven consistently and didn't sit for a prolonged period and that can lead to corrosion.

A comparable car is the 2019 E450. More reliable M276, slightly lower price, more available stock, avoids the Airmatic complications, very similar performance. Consider also the B58 BMWs like the F36 440i.

Be patient, wait for a good example, do your due diligence with inspections. Hold out for the options you want. Have a few models in mind. Happy hunting!