Hi
I have been an XF owner for 2 years now. Basically if you get one with a full service history they are as reliable as anything else. Mine has cost the same to run as my wifes 2015 Skoda Rapid! The only real weekness seems to be electrics in terms of the battery. So many problems you read about seem related to a less than perfect battery. If you are only using for pottering you will probably eventually run-down the battery to a level where wierd problems occur. Solution is simple either go for a long run every week (also helps the EGR and DPF) or get a CTek trickle charger.
If you do choose a Merc I hope you enjoy it, they are nice cars, but you will miss out on the unique Big Cat experience. I know which I would rather be sitting in 🙂
Regards
John