like all forums, you only get the dark side. Nobody comes online to say how reliable their cars are :-) So. with this in mind the disasters are usually caused by neglect or the rare mechanica failure. Mind that every part of an engine will at some time wear out and yes, higher mileage and age will determine price as you say, not necessarily indicating impending doom.
Sticking to BMW's timetable for oil changes is a mistake, gear box oil is not really lifetime, timing chains should be treated the same as belts and again not for life. HPFP and turbos again are bound to wear too. You seem wise to the fact that anything over 50K miles or ten years is going to need someone handy with the tools so prepare for some maintenance and budget.
Parts are reasonable and sometimes shopping around gets results, often BMW prices beat Ebay for example.
If you have a garage, tools, jacks, stands there are not many specialist itmes required (except the timing cam lock tools for a timing chain replacement). I use Haynes and Youtube for most jobs, mostly preventative work.
Diesels have a tendency to be trickier than petrol, timing chain is in the 'wrong' place for example.
For those of a spanner shy nature, these are expensive motors to keep on the road. An indy might charge around £600-800 for a timing chain. Dealership ? no idea as they don't generally want the work so over price it.
Some folk get away with 100K miles plus with no problem.
Cooper 2009, R57.