1. Year model
2. mileage
3. Are you bying from a BMW dealer?
4. How does it sound/smell when cold started (you have to ask the dealer not to start the car in the morning until you arrive)...check the number of start up - cranks
5. second or third generation common rail turbo?
6. small things (aircon; climate control; generally the incabin controls - nudgy/stuck etc...
7. Depending on mileage...but shocks...if 4 years old...shocks proably gone
I have my 120d for 4 years now and simply love it. the most reliable car after my Tazz that i have ever owned.
keep us updated...