At the age of the E53s it is extremely important to have a pre-purchase inspection by a well recommended BMW dealer or independent shop that specializes in BMWs. As important is to find an X5 that includes all or most of the history. An alternative is to look at newer X5s. The increased cost may be more of a value compared to the cost of repairs of many E53s out there. If you are comfortable with DIY that is certainly the best position to be in. Being for your wife, when repairs are needed, there will be downtime so dial that in. I agree with you, if that is what my wife wanted I would be sure she drove one before doing much looking. Some here will certainly be of the opinion that there are other options that will be more dependable. I purchased mine new and mine has be very reliable in spite of many performance mods. I would purchase something else rather than purchase an X5 without at least several recent years of history. If you can find a warranty to purchase that should be a consideration. If your wife is not performance minded a 3.0 might be a better choice than a 4.4.

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