Do your research and watch out for the 6.0 and 6.4 L as they've both had problems and need to be 'bulletproofed'. Prices can vary - cheap if it's a stock engine and more expensive if it has a number of 'deletes', head studs, etc.
I had a gold standard 7.3 that was a workhorse and is probably still running strong. My son currently has the 6.7 and he's very happy with it.
Depending on what you're pulling, don't rule out the Ecoboost. I have one in my F150 and also in my Expedition Max. They really have a ton of power.
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
'17 Winnebago View 24V, '02 R-Vision B+
RVing for 20 years & 200,000+ miles