They will do massive miles if they have been looked after mate. You need to check swirl flaps, not sure if all suffer from this but know certain age ones do.
Thermostate is another item that can fail causing poor MPG
EGR Valves, clog up with lots of town driving
Turbos can go, seen these destroy engines but seems to be rare.

They are great cars, I guess your talking about the E60? Most seem to come with good specs