Welcome to the Forum
I'm not a 240D expert but on any diesel (or any vehicle for that matter) a solid maintenence history is a good indicator of how the car has been kept.
Enthusiast Diesel Owners tend to keep everything related to their baby from parts removed to receipts from Wally-Mart for oil.
Not every owner is an enthusiast, but these cars do benefit from regular maintenance and most people will be happy to show you all the time and money they have spent to look after their car.
Good luck with your search.
