Ford was offering a HD version, warrantied as mentioned. Worthy of consideration.
BTS and JohnWoods are nice, but spending and you said that isn't in your near future.
I'd be hesitant of a used trans. Soooo many variables. I'd go the Ford HD unit. Warrantied at any Ford dealer (or it was that way).
Whatever you do add add the biggest additional trans cooler you can find. Many like the cooler from the 6.0 but there are others out there. Keep it fed with fresh fluid every 30k miles or so and keep it cool are the keys.
I gave up my 7.3 for many reasons, one of which was a trans that was starting to show signs. I would have gone the ford HD route personally. Be persistent if you go that way; not all dealers even know it is available. The group over at ford-trucks.com in the 7.3 forum can help you out.
Good luck!
