Man! Such doom and gloom about this. I know this is an old thread, but if you have a donor car sitting right there it is very possible to do something like this. If you go about the attitude that everything is so hard and discourage people, you are doing them a dis-service as well as other readers. It would be another matter if the OP did not have the M5 to use as a donor vehicle. If both are E39s of the same approximate year, if it fit in one of them it will fit in the other.
I had a 2000 E39 528i and it was a super car, one of the cheapest-to-own cars I have ever owned. Since I bought a E60 535i for my wife I sold the car to my Sister because she had wanted it for so long. Turns out that her Step-Grandson kind-of stole the car and trashed the engine. For the same money as purchasing another BMW engine I can build and install a nice Chevrolet fuel-injected LT1, Balanced and blueprinted, stroked to 383 Cubic Inches that will blow the doors off an M5. As everyone was so quick to mention, there will be wiring, motor mount and exhaust issues.
But for the OP, he would have everything he would need to do the transplant and all he got was discouragement from doing it. By doing the transplant, he would learn a lot and certainly would share the information that he experienced with others on the forum. Why would anyone want to discourage someone from wanting to do something like that?
Jay300ZX, it is probably too late to get the car that you were asking about but if an opportunity like that arises again, GO for it. That is how everyone on the forum learns. Because it too difficult for one does not mean that it will be too difficult for someone else. NEVER GIVE UP!
