BMW does "debug" its cars during their lifecycle. Buying an '04 or '05 will be more reliable since all the r&d work thats been done by people driving these cars since '98.
Also, the later models will have less problems since they're newer and have less miles. They may or may not have as many issues in the future.
My '02 330xi just hit 118k miles and things went to hell this past year.
I replaced 3 out of 4 window regulators (I'm a pro now). I had a shop do my CCV and OFHG at the same time. I replaced my VCG, the passenger-side front axle, my front swaybar endlinks, headlight washer motor and my lower radiator hose and temp sensor.
I also paid the shop to pressure test my coolant systems since I had a leak which I thought was the temp sensor; turns out it was my expansion tank leaking again. I had it replaced 3 years ago!!!
'07 -'11 wasn't bad though. Had the waterpump and e-thermostat replaced under cpo warranty. I paid a shop to replace the FCABs, expansion tank and change the auto transmission fluid.
I replaced the belt tensioner pulley, steering shaft guibo, upper radiator hose, power steering reservoir and filter and brakes and rotors twice.
Things were going very well until 100k miles.
~ '02 330xi
~ '89 RX7 TII