As I had been collecting information on this as part of my build choice thought I'd add this to the postings in case anyone was contemplating a build using BMW 6 cyl engines.
Certain Marlin Roadster can use them and I'm sure that in time more and more Kit cars will if they don't already.
A noticeable weight drop occurred going to the M52 as Alloy was used, the reason that power was apparently not as good out of standard build can be attributed to the German insurance market. This meant that the inlet manifold from the 320i was used and so somewhat restricted the 323 and 328, a little work on changes can give about 20bhp back in this area so do not count them out as basic blocks to start from.
In some cases the engines can be sourced from 5 series and even 7 series but I've limited this list to the E36 / E46 as this I believe will be the more likely choice for Kit building in the future.
Here is the little map of engines and models to try and simplify the vast choice.
(http://img197.imageshack.us/img197/6476/phpfg3jxc.png) (http://www.postimage.org/)