Be sure to ask the person doing the inspection if the front and rear suspension has already been addressed. And it there is any indication of other wear items that are usually addressed around 100,000 miles have been addressed. If he knows X5s he will know where to look. Depending on what has already been done will make a big difference in how much future costs will be.

If you want to keep the stock stereo but upgrade to link iPhone music and some of the other features, I would consider the Dice media bridge. You will have bluetooth, USB, Aux, plus Sat radio depending on the model you choose, all operated by existing controls. If you decide you need better speakers to support your music there are sets that plug and play in existing locations. I have had one for a few years and it works great.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas