I'm needing some guidance/education.

In the very near future, I'm going to be in the market for a f250 diesel.
I've been attempting some research on the net, but I don't know that I'm finding what I'm looking for.
At this point, I know I'm looking for the 199/5 to 2007 generation.
The little bit that I was able to find thus far says that the 2003 and older have the better engine/drivetrain. Seems like the 7.3 was pretty much as good as it gets and the 6.0 had all kinds of problems.
I like some of the options the newer trucks had. The Harley Davidson edition seems awesome. But then again, I've never been able to compare them side by side, and I can't find anything on line that goes into detail the differences between the H/D trim and the XLT trim levels. If it is just a difference of things like "upgraded audio" well, i'd be installing my own radio and everything anyway. So that's not really worth it.
But if the overall interior is worlds different/better, then that's something worth looking into.
Would it be a huge pain in the ass to take the cool stuff from the Harley model, or even the king ranch model and put them into the older 2000-2003 model?

Is the 6.0 as bad as the internet makes it out to be?

With basic bolt ons, and a good tune, what kind of performance would the truck make? MPG? I have seen some places on line that say less than 10 mpg and others that claim up to 25 in the city.

Common things to upgrade/fix/modify?

Any other advice?

Any good sources of info?