Where are you located? Unless you're talking Canadian dolloars, $40K is too much to pay for a base C240.
How many miles would she drive in this 10 years? If it's only going to be about 100,000 miles, the BMW may indeed be more value if you like it. I personally like to drive an equivalent BMW as much or more than the Benz. But, if you're going to drive it very far, that's where the Benz pays off. For starters, BMW's have timing belts versus chains on all MB's. A BMW is a cheaper built car, you get what you pay for.
Also, all models in the US come with power seats and the CD player can be dealer installed for about $600 at any time, and be identical to the factory installation.
The most important thing I could suggest is to ensure that SHE drives both cars.
Best of luck,
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in