I'm not so sure that 7.3L will be worthless in 10 years especially if it is taken care of. It will certainly still be running well with proper maintenance. The other thing to think of is that a CP4 failure in the newer 6.7L truck will cost almost as much to repair as the entire 7.3L truck!
I agree with Gritdog; I would buy the truck in a heartbeat too. OP, the part about a mild tuner on the 7.3L is spot on and will make it feel like a completely different truck. The other thing is that the Super Duties of that era were quite under rated in regards to published towing limits.
Hopefully some 7.3L owners will chime in for you.