I've had many BMWs in my life. As long as you buy a straight 6 it should go for miles.
Highest mileage I had was a 520, which I got up to 290k, had an E46 325 Coupe to about 130k, E36 coupe to 200k.
And ive had a few others too. Never had a major repair bill on any of them. Biggest bill was probably £400 on the 5 series because the auto gearbox gave up and needed new fluid.
So when it comes to BMWs, as long as they have some level of regular servicing (even if its not main dealer) I wouldn't worry too much about mileage.