I am amazed at all the insults we 240D owners receive from our fellow members!!! Too bad they are all true... I own a 240D and would not recommend it over a W123 300D or the 300E. Mechanically a 240D is virtually bullet proof, but so is the W123 300D while providing acceptable levels of acceleration. If you still decide to buy the 240D and it has a manual transmission(I don't know how anyone can stand an auto 240D) then you can do the following to see if the engine is up to par:
1. With the A/C on the car should be able to easily accelerate from 20mph in 3rd gear.
2. It should be able to cruise at 80mph with reserve power available.
Also remember, the mechanicals might last a long time, but the rest of the car is still 18 years.