Look under the car at the rear axles - grab them and shake them and see if they feel solid. Look at the boots. Are they cracked or leaking? Shake the exhaust system and see if it feels solid and make sure it isn't really rusty.
Also, when test driving, how much play is in the steering? If it pulls to the left when driving, what does it do when you stomp on the brakes?
Be sure to look under the motor and transmission after you drive it and check for fresh oil.
Of course work all the lights and the wipers and make sure the electrical system is ok. It it has an ac aux. fan make sure that comes on.
One last point - I know this sounds negative and I don't mean it to be - but usually, and I do mean usually and not always, when you buy a car from somebody, and they don't know anything about the car they're selling you, for whatever reason, usually, you soon wish they had been able to tell you more about that car. You wish that after you already own it, and it's your responsibility to keep it going. If you like working on cars that's OK but just keep it in mind.
You can put an add in the paper and say "wanted clean well maintained 240d" or something to that effect if you are having trouble finding what you want.
If you pay 3500 for an a-plus car, that's a bargain compared to 2200 for a car with problems - because mercedes parts are expensive, and that thousand or fifteen hundred you save up front will evaporate.
At a minimum have somebody check this one out for you. The seller should have no problem with that and the money spent checking is good insurance.
Best of luck,