The S54 engines are solid, they call them bulletproof but the head gaskets are almost guaranteed to go due to the thin cylinder walls. Most common points are between cylinder 5 and 6, but I've seen pictures that show splits between cylinder 2-3, and even 1-2. A friend has a beautiful Laguna Sega, maintained to a high standard with full BMW history and his head gasket needed replacing at ~75k miles. Another friend has a Red one, same again with full BMW history and sub 80k miles, he just had his done last year too. Touch wood mine is spot on however I am going to do a compression test just for peace of mind at some point. There are other known faults too such as noisy vanos bolts etc, but it's a £50k car and most are 10s years old so faults are to be expected. They have less faults than other cars in the same league though imo.