Built for distance
The naturally aspirated 324d prioritises simplicity; the 324td adds a turbocharger for useful mid-range performance. Neither is a modern diesel in refinement or emissions, but a healthy example explains why six-cylinder BMW diesels earned a loyal long-distance following.
Condition before rarity
Cylinder-head condition, cooling history, turbo wear and correct cold starting deserve priority. Diesel-specific injection parts can be harder to source than common petrol components, so a complete, running car with evidence of careful warm-up and cooling-system work is worth more than a cheap non-runner.
Who it suits
Choose an M21 for period character and steady-distance use, not for cheap modern commuting. Fuel savings can quickly disappear into specialist repairs. The attraction is an unusual chapter in BMW engineering and the relaxed rhythm of an old-school diesel six.