OP:

Since you don't pay for downtime, you can't really use the crusher example, except to the extent that your partner has a fixed plan and sticks to it.

You can buy an extended warranty from BMW (not aftermarket) if you act before you have 50,000 miles.

Since you have relatively high annual mileage, you will get clobbered on depreciation flipping vehicles so often. I would try buying an extended warranty to take it to 100,000 on your next one, and plan on selling it with a year left at a more standard annual mileage, ie 85,000 miles. You will get 3.5 years out of it. That way you can sell it with a BMW warranty with nominally a year left on it. Takes the risk down for a lot of purchasers, which will help your resale value.

I wouldn't buy a maintenance plan, I would have an independent mechanic that knew BMWs that I trusted if I wasn't doing the work myself.

I don't agree that the diesel is good for a lot of miles any more than the gasoline engine. But I think both are fine, it is other things that will hit you, whether it be suspension rebuilds, transmissions, or electronics. The engine is the last thing to worry about.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue