I'm not sure how reliable the X5 is after 50k but I can comment on my experiences with my last BMW. It was a 2000 328ci, bought it was 32k miles and drove it til 170k. yup, drove that sucker for 138k miles.

I did most of the maintenace myself. I had to replace alot of sensors, belts, and suspension parts like:

control arms bushings
belts
radiator
ac compressor
expansion tank
thermostat
camshaft position sensor
valve cover gasket
water pump
oil level sensor
oxygen sensors
belt tensioner
radiator hoses
crankcase valve
transmission failure at 145k, lost reverse

I expect I'll probably have to replace most of these items on my recently purchased 2007 4.8 X5 too. Luckily I have extended warranty to 100k.

My advice is be prepared for alot of repair bills from 75k-125k miles. Even after all I went thru with my 328, I still went back to BMW for my upgrade. BMW makes great looking cars, but they come at a price.

I suggest getting some sort of extended warranty for high mileage BMWs.

Good luck