Last time I added up the money I saved NOT paying for extended warranties, it added up to over $20,000. Way more than I have spent on repairs out of warranty....
Expect a fine German machine of mature age to spend $2000 a year on maintenance and repair. Compare that to a car you buy new - there is quite a difference, with the loss in resale value making up a much larger portion.
Mercedes has a tradition: car goes to about 50% of its sale price in about 4 years then stays there for the next 10 or so.... that was in the past. But our Bimmers might have a different trajectory. My impression over the years is that extended warranties and CPO's invite abuse - brakes, transmissions, that sort of thing. The cars feel like you can race them (like Porsches) but they aren't really built for that kind of daily abuse.
I'd say if you actually care for it like the Europeans do - not abuse it like we Americans - and Autobahn it occasionally (he he) it should be a good investment.