Prior or subsequent generations I'd perhaps agree. Not on this one I'm afraid. If it has been sitting at the mech for 2 years it is not going to be in immaculate condition IMHO. I also don't know if everyone realises just how shit the N63 of that era was or how astronomically expensive it is to do anything on it (properly anyway).
You did read how much Jusin paid above to do seals and bearings right? That's the exact refresh you'd need to do on the new motor

... he actually got off lightly. That's an M6 though and will always have value. A 750 will not. Then you add valve stem seals, injectors and timing chains to the mix and you can just go buy a running car if you really want to.
So 100K for a motor + 180K for the refresh + 50K for the car itself...
R330K to get this thing to be barely a project (still in need of detailing/fumigation and with unknown suspension and electrical issues), for a highly undesirable car. You could probably offer one of the guys on Autotrader R350K and they'd take it for one of the 'nice looking ones'.
This is a parts car and most likely has already been used as one to a degree by the mech... whether he admits to it or blames it on his staff.