2002 330Ci here. Do about 2000 miles a year now, previously 10-12. My specialist recommends dropping the oil every other year in addition to normal servicing (which I get done at MOT time, unless a service is due anyway) for a usage that light.
Cooling system will need major attention every 60000 miles or so, so try to pick up one that’s had it done relatively recently and you’ll be all gold.
Manual gearboxes are tough. Autos are “sealed for life”, and enthusiast forums are split between people who say it’ll explode if you don’t get the fluid changed at 100000 miles, and people who say it’ll explode if you do
Diffs can leak.
Rear subframe mountings can tear, this is more common on earlier cars
and will require remedial welding.
Handbrake is rubbish even when everything is new.
they’re quite picky on tyres. Fit a good named brand otherwise you’ll end up in tramlining hell.
If I think of anything else i’ll come back!
I’ve had it 6 years, done 40k miles in it, and during that time it’s coughed me two £400 bills outside normal servicing. Not bad really for a car that’s quick, comfortable and actually quite practical – two bikes in the back of a coupe with care