The naturally aspirated inline-six behind the E46 3 Series and E39 5 Series — widely regarded as one of BMW's most fundamentally sound engines, let down only by cost-cut cooling system plastics.
The internals of this engine — block, head, bottom end — are genuinely excellent, and well-maintained examples routinely exceed 300,000km with no major internal work. Almost everything that gives this era of BMW a mixed reputation is plastic: the cooling system (radiator end tanks, expansion tank, water pump impeller) was cost-engineered to fail around the 10-15 year mark, regardless of how well the car was otherwise cared for.
Not a guarantee, but a pattern seen consistently across owner forums and independent-garage reporting.
Original plastic cooling system components reach the end of their realistic service life, regardless of mileage.
VANOS seal wear becomes more common, causing a rattle on cold start or rough idle.
Core engine internals are routinely still original and sound on cars with documented oil changes.
Ask specifically whether the cooling system has been upgraded with metal/aluminium replacement parts — on a car this age, it's the single biggest factor in long-term risk.
Ask directly whether the car has ever overheated, since a plastic cooling failure can escalate to head gasket damage if driven on for even a short distance.
Listen for a rattle on cold start, a common sign of VANOS seal wear — inexpensive to address but worth factoring into your offer.
Model-specific pages, with notes on how it behaves in that particular car.