Both of our Excursions are running original tranny's. The 00X has 270K miles and the 02X has 307K miles.
As far as throwing money at a 7.3 diesel? Any vehicle/motor 15 years old will take a little money here and there to keep on the road. I had an injector fail the 02X this year so decided to replace all 8 with 175cc injectors as they were less than half the cost of Ford injectors. The 00 X has all the original injectors as does the 340K mile F250.
A friend of mine just bought a 06 F350 with the 6.0 with 185K miles. The previous owner let a lot of stuff go on it so he just dumped $5,500 into it and only a few hundred of that was for the motor.