Thanks everyone. I drove the car quite a bit yesterday and had it inspected at a shop, who turned up some minor things and a possible heater core leak. Then they quoted me $2100 to replace the heater core! I thought that was more in the range of $600- $800.

I brought the car back to the owner and told him what happened (showed him the report) and he called his own garage and got a ballpark of $600 to repair.

I probably shouldn't have driven the car around quite so much, because now I really want it and am not objective anymore. :-)

He has 2 other cars (2006 e350 and a Jag) which is why he is selling.