So Diesel: Go for M57 (if well maintained), avoid N4/57.

What about Petrol engines? The 4 pots wouldn't be of any interest, so re: the 6 pots - what are the ones that proved to be reliable, and the ones that proved to have design faults or common problems?

Edit: Wasn't just talking about engines by the way, cars in general. It's no good if the engine is healthy and the rest of the car falls apart... Heard the 335i have HPFP, water pump, and turbo issues?