Had a number of cars that needed significant welding up after floorpans rusted through in our early car owning days, but, the worst car I had was also in some ways the best - An Audi A4 V6 2.5TDi. Dynamically, a brilliant car and significant motorway eater. The problem was it cost an absolute fortune to keep on the road. ECU replaced (thankfully under warranty), Door locking pump replaced (after the dealer broke it an wouldn't own up), Turbo Charger replaced (Ouch) and then the crank-case breather pipe blocked up immediately afterwards, Timing belt cost a king's ransom to replace, Fuel pump, the list was endless.