One word, electronics! Plus maintenace, the A8 is cheap for a reason, nobody sane can afford to keep it going (either by paying a shop or DIY). Certain parts are crazy expensive too. Hats off if you can make it work, but remember these cars have lots of electronics that are integrated, take away the engine and perhaps the gateway goes down (so nothing works-even power windows, radio, abs, seat heaters its all integrated). Its a great ride, but modern Mercedes have caught up with diesels.