I work on alot of these cars everyday, (E46) , They all seem to have the common problems that are described above, suspension is of course very important in BMWs, as we all know, one bushing can make a world of difference in handling!.
The mechanical part of the drivetrain holds up well, it is things like water pumps, t-stats, cam sensors, MAF, O2s, catalytic converters, abs sensors, abs controllers, the PCV vaccuum valve, failure of the vanos piston seals, expansion tanks and level sensors. I recommend having the all the fluids done every 60K with the exception of oil of course. Brake fluid flush every two years.
The Body throws another realm of parts into the mix, besides numerous modules that fail there are the window regulators, suspension components, brake rotors, rear springs, etc.
I work on alot of minis too, and have been replacing alot of low mileage 5 speed transmissions to the tune of $4K, also rear brake calipers, rear rotors nearly impossible to get off here in maine!, crank sensor o-ring, oil cooler gasket, just had an instrument cluster fail, LF ABS sensor common, drivers door latches, etc.
Overall I think BMW is a good product and in the long run if you want something to last a long time, you have to maintain it, and take care of it, in the long run you will have a fun car to drive.
Mini on the other hand can be fun, I do'nt think of them as a good maine year round car, due to thier narrow tire foot print they can sometimes handle squirrelly on rutted roads,
I dinanized a 2005 miniS conv, and it flew!.