From what I understand, the 6.4 had problems just like the 6.0. Did yours have issues? I blew the head gaskets at 60k miles. I have a few 7.3L in my fleet, and I'll never get rid of them. One of my 6.0L I just had to put about $2k into for random bullshit. I said I'd never buy another ford diesel again, yet here I go. 
I was just gonna post asking a similar question about longevity
I only had mine for 120km and I only modded it at 80km or so. I beat the bark off it. Ran it down the quarter mile many times. Beast got 26or so mph. With a light foot on highway
Prior to modding it I was getting 10mpg on average.
I never had a single issue with the truck or driverrain
Would def buy another
Although I have heard of issues