Very interesting debate. Because let's put it this way, I personally will never buy a car that I know or suspect has been modified! As we all know, tuners can work miracles but many can do the opposite and wreck perfectly reliable cars!
But certainly a car bought while still under motorplan one would expect and hope that they are completely un-fiddled with! Older cars out of motorplan is obviously a different story and up to the buyer to investigate and pick up on these things!