I have seen cars with these kits before, they are basically parts off a S220, modified in some way to fit the 140, I think they look cool, but I dont recomend doing it either.
A trained eye can tell once they get inside the car, or when they look at the trunk. A 140 and 220 interior is totally differnt so is the size, body style in the back, etc. to do a full conversion would cost a fortune.
I guess you could fool people who dont know much about mercedes, but like you said, hobbie is ok, fraud is not.