I remember when the E36 M3 came out and many said oh it will last maybe 100K. Yeah right. Newer engines run so well for so long especially with terrific lubrication that 300K miles is conservative more like 400K miles is when I would expect failure of a major component. My friend who is driving my 284K mile BMW M3 convertible has put thousands more on it now. Transmission should go well past 200K miles if you change the fluid and filter every 50K miles.
Fix what breaks and nothing much other than typical failures like window motor wiper motor broke on the car.
You will be long tired of the car before the engine fails unless you don't pay attention to a water pump failure which would/could take the head gasket out.
As mentioned above periodic maintenance is essential to long life of a BMW. You can run it 100K miles and do nothing but you will pay for that in the next 100K miles...