I personally don't think the BMW 3.0 engine is anymore specialist than the BMW 4.4 for example?
Also if you have the Rover 3.9 or 3.5 V8 the chances of getting it serviced - or re-build (more like now due to age?) "correctly" could be harder than someone looking after the PlusSix engine, as most service engineers disappeared years ago?
Mark
Honesty means doing it right, even when no one is looking!