While this is the perfect place to ask your question, I cringe when someone asks advice about buying an X. I expect they will be bombarded with a three pager of all the things that will go wrong. I can't tell if they are summarizing every problem anyone has ever had, an X owner that if they were logical would have cut they loses long before now or if they are just not happy people and want to share with others. So far you are getting good input.

As a rule of thumb, the suspension will be in the how long do you think it would last, bozo, somewhere North of 100,00mi. I must have my X handling the best it can so I replaced all wearing parts on both ends then. All the parts had many miles left on them but I could sure tell the difference. I Dinaned up the engine and aftermarketed my suspension when I purchased my X new. At around 95,000 miles, the fool that I am, added 150 shot of nitrous. The point of all that foreshadowing is not the problems I have had. I have the original trans, water pump, alternator, fuel pump, air bags, etc. etc. This is the first in a long line of vehicles that I have kept more than 3 years. I have never been left stranded and it has always been my daily driver. I have no blue smoke and it is only about a half quart low when I change the oil at 5000 miles.

All that gobbledeegook said:
There are some 3.0 forever folks out there but I agree with Term, go 4.4 for the torque and 6 speed for the towing, mileage is similar anyway. I would see if I could find one in the 60,000 mile range that there is since new history and , of course, do a PPI. That way you have insight as to how it was used, driven and the environment, which as you know, will influence when sh*t MIGHT happen.

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