Well maintained, I don't think 500k is unreasonable. The trans will go before that and you'll probably have to do injector seals, but it's a pretty damn reliable engine otherwise!
I am at 232'000 and counting, on the 2000 Excursion 7.3 that i drive everyday to work and out west and up north for riding, i guess i am aiming for the 500'000 mark!
Had the truck since new and regular maintenance done, i have spent less on that truck than my wifes 2005 Ex with the 6.0 that is at 100'000
I have a buddy that has had 3 go over 500,000. I am at 140 on 2000. Never had a problem with it... Change all filters when you change oil and it will run for ever.. I have been running amsoil now for around 50k... Love it!