The powerstroke 7.3 was introduced in the mid 90s and ran until 2003 with a big body style change in 99. You want a "late" 99 on up to a 2003 with the 7.3, the best year is a 2000 for a couple of subtle reasons.
The 6.0 liter engine came out in late 2003 and was/is a dud. Major issues and not worth the risk. Sure, some later 6 liter engines were fine but the majority have been extremely costly to keep running. When they do run, they run great with good power. When they fail, it is very expensive to fix, as in many thousands of dollars. 6.0 diesel trucks can be had for very cheap due to their extremely poor track record.
You really can't beat the ford truck quality from the 99 model year on up in supertduty trucks.
In all of my years reading about the 6.0s on various forums, diamondpilot is about the only guy that has been happy with his 6.0. I trust him and believe that he has a combination of very good luck, excellent maintenance, and he properly uses his truck in such a way to keep that 6.0 happy.
Oh and one more thing. Don't get a 3/4 ton. Get the one ton non-dually. This will give you an additional 1000 lbs of legal cargo capacity with no negative effects.
I bought my 2000 Crew cab F350 diesel from a craigslist ad that a small dealership had posted. Searchtempest.com allows you to search craigslist ads across the country.