As a 6.0L owner I too questioned all the same things. We originally bought a V10, but soon realized in a towing situation, the v10 is just a dog for the NW of America.
That said. I 100% agree with the other poster. If you see door pod gauges, any mention of bully dog tuner or the like. Run AWAY. You don't want a 6.0L that has been "tuned", UNLESS, they did the head studs and EGR delete.
Also, for towing, I would by an 04 or newer with the torque shift auto. It does engine braking and works very well.
My truck has near 200K miles and no real issue as long as maintenance is done. Keep the coolant changed as it should be and best to eventually do an EGR delete and oil cooler move. The EGR delete will require a re-tune on the computer but it is only for emissions. Check your local laws first, places like Oregon have emissions on diesels but only under 8500gvw, ford f250/excursions are above that rating as in most states. If you live in Ca. well no help there.