WHERE did he say he was looking at NEW twucks of ANY sort! Only USED was mentioned. So the 7.3 with a 10 sp auto, is not an options from what I see!
That said, 6-12% grades......those are a piece of cake, try some of the 15-25% grades in off highway situations! Interstates a easy! its in the camp grounds, local roads, roads down and back up river canyons etc that you can not make it up due to too low of gearing etc. Be it a gas or diesel!
With that said, get the best truck you can find, and head out! It will generally speaking take at todays prices of the difference, fuel cost etc, over 120K miles to pay for said diesel. How fast will you get to that said break even point?
Yes, ANY motor with a turbo/supercharger on it, will be better at elevations above 5000', otherwise, yes, you will lose 2-3% of the sea level advertised HP per 1000' of elevations. so at 10K', you are down 30%! How long, will you be up that high to really care per say?!?!?!
Marty