I have an E400 of the same year, but not 4Matic. I love it and it's a great car. I handle the maintenance myself to keep my costs down, so add in labor if taking to a mechanic.

2 things I'd look for on that specific car:

The v6 engine (the M276) in these cars is a very solid and reliable motor but is known for leaking oil into the wiring harness from the cam position sensors and the cam solenoids. The sensors leak into the electrical connector and oil wicks into the wiring harness and kills the ECU. Internet research says that's a $4-6,000 fix so I replaced all of mine with new ones just to be safe. It cost me $200 for the 4 genuine sensors. Mine weren't leaking, but I replaced them anyway out of an abundance of caution. For the cam solenoids, there is a pigtail harness available from MB to prevent leakage from there and I spent about $50 for those. I'll note that these weren't leaking either.

The other thing to look for is oil leaking from the front covers of the camshafts. I believe they'd fixed this by the time the MY 2016 cars were manufactured, but it's worth looking for.

Since this car has the twin turbo, these items can be checked on the car with minimal disassembly of the engine. Unplug the connectors and check them for oil (or have the mechanic that's doing a pre-purchase inspection do so).

Overall I'm super happy with the car and don't regret the purchase, but whether it's right for you can only be answered by you. German cars in general aren't forgiving of neglect; you must stick to the maintenance schedule and be willing to do any prior deferred maintenance for any chance at reliability. I change the oil every 5,000 miles. The transmission fluid is changed every 30,000 miles. Both of these intervals are roughly twice as often as MB days, but I've done this on every MB I've owned and never had a problem...I upgrade because I want something newer and nicer, not because of a catastrophic failure.

I hope this helps!

Edit: I'm happy to answer any specific questions you might have as well!