If you spend time in Germany or with Germans you will understand they have little tolerance for not doing things the RIGHT way. I've owned over 20-something cars, and quite a few of them were 6-cylinder BMWs- I've never really experienced the misery people talk about online regarding the brand but here are a few things I think help:
1) BMW does 6 cylinders exceptionally well, for the most part, try to stick with those engines and ideally a manual transmission. Right there you should have a very robust drivetrain. The ZF8 though is a great transmission alternative and their Getrag DCT's are pretty reliable.
2) German cars generally use more environmentally friendly plastic in the engine bay, so treat cooling systems as maintenance. 7-10 years give or take. This is a classic example of how being proactive instead of reactive will save you A TON. Family friend ignored my advice and blew their M62 V8 on a road trip in their 2001 740i- that was an expensive lesson for them.
3) Only use Original Equipment/Genuine BMW parts, if you do research you can find the cheaper version of the original equipment part typically with the BMW logo scratched off. This is another area where I think people unknowingly suffer, they take their car to a mediocre shop that uses the cheapest OEM parts (and there are plenty of crap OEM parts) which results in continuous problems.
4) BMW's aren't Toyotas. They aren't built for whatever fluid works, hit-and-miss servicing, and general appliance usage. Also, service the transmission and differential(s) even though most newer ones don't mention it in the service guide.
5) Don't buy a crappy example, it will burn you badly. Also, having a local indie shop that REALLY knows its stuff makes your life much easier. I'm fortunate to have a friend who owns a predominantly BMW shop (though they work on everything), and he is like Rain Man when it comes to knowing every issue, every part change, which part year to use or not to use, etc. If you don't have that then online is your next best bet, but I also see a lot of incorrect info too, so there is a bit of a learning curve.