Wow and double wow! You are the Man!...anyways, I believe every syllable here however, what I think your (the X5) experiencing is the domino effect (one main problem, fault, malfunction) leading to another light or fault code and so on. Address the main one or two and your...
"on the road again"
I don't know what the car's condition is but I suspect you attend to it as needed and then some. It would be a too good parts car. Selling it as is,,,you'd get more satisfaction driving it into the ocean, sorry...off a cliff.
Best you, your not going to like this...
Patch it up reasonably and make an attempt to then sell.
But I like the idea of figuring out the problem and DIY in correcting the problem but moreover, I fear you don't have that kind of time to devote to the X5....bumme

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