1. Exhaust manifold leaks (more evident on cold start up) they crack near the exhaust flange on the engine, manifesting itself as a light rattling noise from the top of the engine that disappears after a minute or so. The rear passenger side seems to be the most common point of leakage. Easily repaired by welding up (unlike the v8 Ferrari's they used better metal on the V12 manifolds) but still budget around £500 to get it fixed.
2. Camshaft oil seal leaks look around the backs of the timing belt covers, a slight bit of damp is normal, but if it's gushing oil out factor in £1000 plus parts to put it right!
3. Rattle from front of engine at idle; timing gears inside case worn, a massive, expensive job that should see you walking away and finding another one!
4. Knocking from suspension over bumps, standard knackered ball joint noise but each corner you hear a noise from you should budget £700 to do top and bottom joints (worst case!) into your offer.
5. Flickering dash display. The TPFT displays in these pack in and cost an absolute fortune to replace. Look for any flickering in the display panel or green-ish tint to the display as being early warning signs of failure. A £1700 bill.
6. Seat heaters. Check they aren't stuck on! The control down the side of the seat packs up, not massively expensive, but one to look out for.
7. Aircon not working. Could be a few things but commonly the AC compressor leaks between the two halves. They are around £1300 fitted, but there are companies who rebuild them for much less.
8. Transmission oil change. Make sure there are records of regular servicing including oil changes and brake fluid changes. Don't buy a car that's been skimped on with its servicing (even when it's a low mileage car)