little help... 08' Powerstroke 6.4L Diesel eval.

I was going to but a 6.4 from my son who sells for a Chrysler dealership....it was a used truck in beautiful shape with under 90k miles. I talked to my nephew, who is a certified Ford diesel master mechanic. He said to walk away from a used 6.0 diesel, but run like he!! from a 6.4. He rebulids them all the time and said they cost about twice as much to rebuild as a 6.0.....the 6.7 is supposed to be the NEW 7.3 in terms of reliability.....i would start running.