"My sister married a diesel mechanic"
Why don't you get that guy, since he would be the one fixing any potential issues, to look at it and help you decide.
People always say to buy the 7.3 and stay away from the 6.0
But I've also heard that the newer 7.3 have more electronics on them which makes it more complicated.
I've also heard that is difficult to find mechanics that know how to work on those older (non electronic 7.3).
Im not a car guy, so in my case, regardless of what I buy, I will either have to learn how to fix it or pay someone else, specially if is a diesel.
not sure if any of that helps, good luck
the more i learn, the less I know what to buy . . .