Mine is a Year 2000 (01/2000) 4.4i.
When I got it at 97K it was dealer-maintained by the book.
Prior to 97K I learned it had the following:
Alternator replaced (the water-cooled one)
all 4 window regulators, then the usual brakes and such.
After I got it I did the following:
Water Pump & Stat, Coolant recovery bottle. Rear shocks.
It ate up a left rear tire in short order, leading to suspension control arms, bushings & ball joints all around. Then new tires. Did the Powerflex upgrade on all the bushings at the same time. Very nice holding onto the road now.
Big mistake at 130K. I decided (after much encouragement from the dealer) it was a good idea to change the transmission fluid & filter. What transmission does not want to run on fresh clean fluid & a new filter? The ZF trans in the X5 after 100,000mi. When they say lifetime, they mean lifetime. There is a reason why there is no dipstick. Mine went South 3,000mi. after the "freshen-up". Check out the other info on the ZF trans in this & other Club websites. It all points to the same outcome. My new transmission (had it rebuilt by 1st Stop Trans in AZ.) runs flawlessly - maybe better than new since its supposed to have a new-generation valve body in it now. The shifting is smooth and predictable in all conditions.
Today it has 137000mi. When the trans was out the rear main seal was doing its job, not a drop. I am getting some leakage at the valve covers & possibly the valley pan gasket. I see an oil diverter replacement in the future.
The parts are spendy but compared to what? Nothing compares to the quality of the drive.
The BMW is all about preventive maint. EXCEPT the ZF transmission.
I suggest you run it untill it goes no more.

Best regards,
SkidmarkX5