Sierra and now MX-5 are the two donor where all the kinks have been worked out. For MX-5 there are two separate designs of the chassis and suspension mods documented, and there is a lot of know-how on this forum. BMW-based cars are more just a subject of rumours. There might one or two built, but we don't know much about them. But if you feel confident enough, go for it. If you post your experiences here, it will be very useful for other builders later on.
That said, one French gentleman designed a full BMW donor version of Haynes Roadster in Solidworks over here, you might find it useful:
http://www.haynes.co.uk/forums/showthread.php?t=3265