Check out a few of these video's too, so you can have a better idea of what to look for when you see them in person.
https://www.youtube.com/watch?v=hvRlRj06A0s
For $6000 the car should have no issues like this car does:
https://www.youtube.com/watch?v=xuepVGnU3UM
Every hose and rubber bit should be new, soft, and supple. The rear drive shaft boots should be very soft and no signs of cracks, the rear fuel hoses should all be new, etc. Otherwise it's a $4000 car in my opinion.
I also would not pay $6000 for one of these unless I had a compression test and coolant leakdown test done by a third party mercedes mechanic (or Diesel mechanic at a Bosch centre). For two hours labour it could save you a real headache.