I am in the exact same position as you are and I thought about posting this same question. Any American engine is not a good way to go for sound because American engines have an archaic loppy grunting sound that are in stark contrast with both the exterior of the Murci and the actual sound of an authentic Murci. People will know a Ford or GM engine when they hear one therefor ruining some of the Murci replicas appeal.
As for the BMW V12 it sounds horrible at idle and sounds more like an engine built for utility reasons. It's not a good sounding engine at all.The BMW V12 engine is extremely underpowered and I believe that the only appeal to that engine is the fact that it is a 12 cylinder engine therefor having something in common with an authentic Murci just like people have placed Jaguar V12's in Lamborghini kits. My comments are pertaining to the BMW engine out of the early 90's 850 car and whatever else it was placed in. That is the inexpensive engine that is most used. I can't comment on any of the latest BMW V12's which will cost significantly more to obtain...
The best option in my opinion is the Audi 4.2 V8 engine. Depending on what year and model Audi you get it out of the horsepower ranges from about 320-450. The Audi engine has the smooth idle and sexy revving characteristics of an engine you would expect to be inside of a Supercar. That BMW engine has engineering issues that will have you at a mechanic far too frequently.
Most people use Audi transaxles in their Lambo replicas. The Audi 4.2 V8 bolts right onto the Audi transaxles.
Video of the sound of the BMW V12 placed in a Lambo kit car
http://youtube.com/watch?v=cWGlq-hcO0oVideo of an Audi 4.2 V8 placed inside of a Lotus with an Audi transaxle
http://youtube.com/watch?v=581rF3HhKAMWe all have our preferences...
