Mine just rolled 150k (2004 4.4i) and it runs/drives like a brand new car. Sure it has a few minor leaks and I've spent some money replacing things like the alternator, hoses, belts etc..

So far it has never left me stranded, aside from the time my alternator went, thankfully in my driveway.

They can be rock solid trucks if you really know what to look for.

Obviously look hard for leaks, make sure the trans shifts smooth, no noises from underneath (transfer case chains, guibos, trans etc..)... If it has the self level suspension cycle it up and down to make sure it works and give the air bags a squeeze to make sure they aren't soft.

Panoramic roofs seem to have problems on almost every x5 I have seen. Open, close, vent, close etc.. to make sure all functions work properly.

Jack it up and shake the wheels around, control arm bushings and stuff tend to wear fast and can make driving less than fun when you start getting wobbles in the steering wheel and stuff.

Finding someone with a BMW coder/diag setup is a MUST when buying a used E53. Make sure there are no major faults stored.

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Current Bimmer
2004 X5 4.4i Sterling Grey Sport/Premium

Past Bimmers
1991 318I Alpine White
1995 740I Alpine White
1991 525I Schwartz
1998 323IS Scwartz
2004 330CI Cabrio Titanium Silver Metallic
1995 540I Schwartz
2000 Z4 3.0 Titanium Silver
2000 330ci Coupe Titanium Silver