Looking for A8 engine and timing chain advice

I have a 2016 A8L 4.2 with 120k miles on it. I am becoming increasingly worried about the potential for timing chain issues. I understand if the guides or tensioners fail, it could lead to major engine damage.

1) Should I be worried and should I have the upper guides replaced, which are accessible without removing the engine?

2) If i decided to replace the car with a newer A8L, 2014 to 2017, is the 3.0T a great engine with longevity but will have similar concerns with timing chain and concerns with the the turbos going out? I would imagine the pricer 4.0T could potentially have additional upkeep costs?