Sounds like a nice X5, that you’ve already put a chunk of money into so what’s the harm in keeping it for a while. At least through the winter where you’ll want something like it for your weather, and then evaluate in the spring or summer. Mine was my wife’s daily driver from 2002 until 2015, and then my kids thrashed it for five years. I repatriated it during the pandemic as I needed a project so I wouldn’t go nuts not working much. By far my favorite car I’ve owned now. It has taken a pile of money to get there though, and like any older sophisticated car, it’s a beggar - always wants something. And that stuff is on top of the mods I still have on my list. Sounds like you are no stranger to keeping up middle and older age cars though.

I’m not sure what the 3.0 gets normally, but I’d kill for that MPG! If it’s on the low side though you could do the 02 sensors. I’ve read here that they can affect MPG as they fail/age. I think you will need to reset the adaptations after replacing them.

No DSP makes it much easier to replace the head unit. I put a Kenwood into mine that I’ve been pretty happy with, and just added a 10” kicker powered sub to the right rear cubbie. The angle of the dash makes me wish the kenwood was a bit brighter, and I wasn’t smart enough to go to the next model up to get HD radio, but it’s light years ahead of the stock system. I’m going to fab an MDF surround for it and learn how to cover it in matching burl veneer.

Have you checked the front drive shaft transfer case splines? If not I’d recommend doing so if you hang onto the car. Depending on your AWD model it can be an absolute showstopper if they fail. In some molds the AC drain line(s) drip onto the splines and corrode them, and/or cars that live in places with bad winter, salted roads, etc. might have problems. Mine were were showing plenty of wear though and my car has only ever been down south and in California, and my AC lines were offset from the t case from the factory. At some point BMW used a longer shaft, and moved the AC drain lines a bit so they wouldn’t drip on the drive shaft end. You may have the OE longer shaft already though, or adding longer splines isn’t a tough job.

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Current
Original owner 2002 E53 X5 4.4i to 4.6i swap
1993 Mopar 318 Jeep Grand Cherokee - original owner
2015 V6 Jeep Grand Cherokee - the nice car for my wife
Former
1972 Audi Fox
1986 Saab 900S
1996 BMW Z3
1998 BMW E36 M3 Sedan
2004 BMW E46 M3
2006 Audi A3 Quatro