Understand where you are coming from, I can see that, but how is an older car that has probably the same milage that I buy all my diesel engines on, with the same "years old" from the current year, going to cause problems with them have things like regular service history, stamps, official BMW stamps what ever.
Surely just because its a BMW Diesel it doesnt get to a certain "age" x years (forgetting milage) and think, ok I am going to start going wrong now.
I recall people saying the same thing about my MK5 Golf that I bought 3-4 years ago on a 04 plate 70K on the clock and it hasnt missed a heartbeat, infact the only things I have had to get done are consumables, tyres, breadpads, disks, and on 146k now the ball joint on the right hand drivers side.
What problems might a what you would call older (2006, 2007) 318D or 320D with a buying milage of 60-70k on the clock run in to that will become disasterous to the wallet, as it sounds like engine problems if its that bad