Hi Steve
I had the ECU die from corrosion on the connecting pins it couldn't be repaired the answer was to change the ECU for a recycled one from another X5 4.6is the immobilizer stopped me short no amount of tinkering would get it going in the end I brought the immobilizer, key, drivers door lock and ignition barrel from the donor car put it all in and it fired first time. Hence my words of caution.
I thought the 320d came in 2 flavours, 150 bhp and 177bhp but not being a Diesel man I am on thin ice here.
Nothing wrong in being wary of putting your hand to deep into the wee dusty sporan, keep turning the pages I am sure you will find what you want. If you aren't aware in realoem if you click on the part number it will bring up a list of alternate parts and models they were used on.
Dave