As a new owner, I'm still eagerly learning the ins/outs of X5 ownership. I can tell you that I HAVE learned a few things that permit me to be more at ease. For one thing, some of the costs quoted in posts reflect exhorbitant BMW dealership pricing and maintenenace intervals that pretty much involve "checking" things. Eliminate the "checking" aspect of these service intervals and costs go WAAAAY down. It would appear that X5's have some exotic issues, but I can tell you that it is not unique among vehicles (my Powerstroke diesel F350 is a good example).

You mentioned transmission fluid. According to BMW, that is a "filled for life" fluid that "requires" no service. Whether you change it or not is up to you. Personally, I would not be in a hurry, but will do it myself when the time comes (it's easy).

At 100k, the long life plugs will require changing. The coolant should also be changed out (I used Zerex G05 which is phophate, amine and nitrite free).

The check engine light could be an issue and needs follow up. Could be as simple as a bad battery or as bad as a ......well... pick your worst scenario.

Before I bought our X5, I Googled and Ebay'd several components (alternator, filtes, brake pads, battery, etc), to dtermine potential maintenance costs, and was pleasantly surprised to find that they were not more costly than most other vehicles I've owned. HOWEVER, buy from BMW and their 300% gouging, and that goes out the window.

Good luck with your decision. So far, I have not regretted mine.

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Cheers!
Chris

03 X5
02 Acura
88 Mustang GT Ragtop
88 Campion Allante 195
99 Itasca 29