Anything that is easily removable and expensive / impossible to buy - especially if you intend to buy another.
i.e. seats, carpet (tend to go for around £100), hcv, resistor pack, stereo, headlights, rear lights, stereo, door mirror covers, front grill, if the door seals are in good nick - take them as well (like rocking horse shit to get hold of, unless you are loaded (Ford £90 each)), etc.
Certain parts when I was doing up my Puma were either very expensive or impossible to source.