There are a lot of opinions on this topic - Surprised no one has jumped on it.
So...there is the older 6.9L (non-turbo), the 7.3L IDI (non-turbo), the 7.3L DI (turbo), the 6.0L, the 6.4L and now the brand new 6.7L.
Many argue that the 7.3L DI is the most reliable, longest lasting of the group. Don't know how much you've read, but there is a lot of unfavorable comments about the 6.0L and its issues, but many will also say theirs has been reliable.
I would check places like New Cars, Used Cars, Blue Book Values & Car Prices - Kelley Blue Book
-OR- New Cars, Used Cars, Car Reviews and Pricing - Edmunds.com
to see what you can get for your $$$. Probably something in the 99-03 range for a 7.3L and maybe something in the 03-04 range for a 6.0L. (03 was a transition year. Started out with 7.3L and ended with 6.0Ls)
Hope this helps - I hope others will jump in too. Good luck in your search!!!