I hear you. All of those are valid complaints about your vehicle. And some others have had the same ones. Hell, I bought a 4.4i that 3 shops could not get to stop misfiring/idling strangely despite thousands being spent on it.
I bought it for parts. But Mercedes (and Cadillac) have had engines with head bolt failures, in really high numbers in Cadillac's case. That's just an example, all brands have their faults. Porsche glues coolant pipes into the engines and those fail eventually.
BMW survives because we aren't their target audience. The people they make money off of have no problem paying $50-100k for a new car. Or are willing to pay thousands of dollars to maintain one after the warranty period. Those are the people BMW caters to.
I don't mean to defend BMW or dissuade you from any other marque. As mentioned, I just get a little amused when I hear on the internet that BMW's are horrible cars. I don't mind it, rhetoric like that keeps these things cheap on the used market so that people like me can afford and enjoy them. Yeah, they're frustrating and expensive sometimes. So are kids, jobs, wives, friends... etc.
Sorry to derail your original question. Looks like it should be easy enough/cheap enough to find a motor out of a 7er if you can't find a 4.8 from an X.