Sounds to me the guy has few takers and has a fish on the line that he is playing for information. He knows you know diesels better than he does and is trying to fix all the untold troubles that keep this car in his drive and his bank account short of a few grand.
Now do you really want to buy this car or just want to drive him crazy. What an opportunity to make him crazy! But I know you wouldn't do that, now would you.
Seems strange that he still has the tiny pipe on the engine he wants power out of. Is he thinking this out or not?
Have fun with it. Let us know if he calls again. My guess is he has your number and that you're his knight in shining armor.
DAS