The only guy I've seen who mentioned he had a turbo'ed M50 engine was a guy name Myron, but he had a supercharger on his car. You might try e-mailing him at myron.marcyniak@worldnet.att.net Don't know if this is still a good e-mail address for him and I don't know if he has the car anymore. He was trying to sell it a while back. I questioned him about selling the supercharger separately, but he wasn't interested.
Here's another board where you might try your question.
http://bimmer.roadfly.com/bmw/forums/forcedinduction/