"If your car is a company one, a letter from a director is needed stating you can take the car abroad." - this is more for when you try to bring it back that take it, isn't it?
Paperwork other than driving documents - as has been mentioned before:
Passport
Camping tickets
Entry tickets
E111
Green Card (mine always comes with a useful leaflet to complete in the event of an accident, I have a small supply of these in the glovebox as so far not needed - I'll regret saying that)
Private medical insurance certificate and emergency numbers
European breakdown insurance
ACO membership card
Maps
Credit cards and money
and a spare set of car keys (yes, Sebringers, just in case you lock yours in the boot...)