Looks like some of your issues were related, and perhaps the first repair only addressed the symptom, not the problem. Battery and voltage regulator were likely linked, same with the three throttle issues. Radiator and expansion tank may have been linked as well.

Reliability can be fairly assessed on a new car with respect to design and manufacturing issues. Lack of reliability on six year old used car has more to do with previous maintenance practices than the vehicle design and manufacturing, IMO.

I don't think BMWs are known for poor reliability. But then I've had five BMWs and have never been stranded once. They are known for not responding well to neglect.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue