Damn fine ride you have got there! i agree to sticking with bmw, but then i would wouldnt i? haha get one of those south african 3.3L M30s if you can? that said dont butcher a car that has one in it tho! those things are rare! you might be able to find one on ebay or something?
Have you worked out your total weight saving including everything.. spare tyre and rear seats as well?