I dont see a reason to remove the fenders, they are out of the way anyway. Some time ago someone changed N62 in E53 with one from E60 I believe. He had to replace oil pan, exhaust and other anciliaries, but it worked. You need to do some research, ther emay be no differencies at all. The engine from X5 will be most expensive and one from a 750i will probably be cheapest(these cars are cheapest to buy salvaged).