I've run lots of numbers on this. Even with major system refresh and rebuild work, my take has been that keeping and maintaining beats selling and replacing, at least with anything else I'd want, by a lot of dollars. Hell, I might be saving enough to pay for that supercharger....

The problem you have to deal with is timing and human emotion. Instead of a nice, predictable monthly payment with the comfort of turning over your problems to a dealer and a warranty, you get unpredictable failure events. That's emotionally difficult to deal with. The $2,000 repair bill you get surprised by this month somehow causes a lot more angst than four $500 car payments would have.

What can you do? Others have made good suggestions here. Keep a designated cash cushion for your car and keep filling it up as you go along - that's the "put the monthly payment you would have made in the bank instead" strategy. Works pretty well, if that's what you need to do. I've been able to just roll with the surprises and have reconciled myself to not getting into a twist about it, I'm fortunate that there's enough cash cushion in my life that I can do that.

Another is to do the work that's going to need to be done before it's needed. Everything is going to wear out. You can replace it when you want, or when it wants. I've cracked 110,000 and am working my way through everything with that approach. Cooling system is fully refreshed, suspension and steering refresh is nearly all done. I get bonus points for the suspension and steering work, since she drives and rides great now that everything is new and tight.

Anyway, good luck with your decision.

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2013 X5M - Sapphire Black
2006 X5 4.4i - Sapphire Black
2002 M3 Euro - Alpine White
2003 M5 - TiAg
2003 M3 - TiAg
2010 535i - Deep Blue Metallic
2007 530i - Deep Green Metallic