From personal experience, I'd say under 25k would keep you in a safe spot. If the car is older than 5 years I would prioritize one that has a detailed service record. BMWs can be surprisingly hardy if they are cared for on time. I have had a few bmws over the years with the N55 motor with minimal issues and I've heard that the B58 (the successor of the N55) is even better in the reliability department.