I would stay away from the 6.4 - the emissions on it is a real problem. Most Ford guys will say go 6.0 before 6.4. International had a real issue with emissions....... the 6.7 is awesome - 2012 or newer. The 2011 ones broke off glow plugs in the cylinders and had manifold issues...
The 6.0 isn't something to be scared off - as long as all the maintenance is done and the proper fixes have been in place. At this point, that should all be done, as these trucks are getting up in miles.
Ours has nearly 180k, and I sleep just fine, I have 0 worries about it. The steering is miserably all over the place but that's a live axle for you - I wouldn't expect the Ram's to be any better. The GMs will obviously steer and front end will ride much better due to the IFS.
None of em are great honestly. the Ford 6.0 and 6.4 are trouble prone, but the truck and transmissions are really solid.
The Duramax/Allison combo is great...except the Duramax's in the mid 2000's ate injectors, and they were even more expensive then the Ford ones! Like $4800 for all 8. The LPL apparently is a MONSTER ENGINE, but newer trucks.
The Ram has a great motor, but the transmission/truck fall apart around it. As mentioned, the Aisin Transmission fixes half of the problem, but thats only on the newer trucks.
I went a properly repaired 6.0 Ford (Obviously). and its been fine, though I do have to run a $30 friction modifier in my oil....
Honestly, if I had to do it again...I'd lease/finance the newer trucks. They really are a serious improvement over the mid 2000's for any of the manufacturers...they just got damn expensive, thats all.
FYI if it helps, I bought our 2005 King Ranch 6.0 Fx4 w/150k for $14,200 - with the EGR/Oil cooler etc work all done.
2017 F-350 6.7 Diesel, CCSB SRW - formerly 2005 KR 6.0
2018 Alpine 3660FL - formerly 2005 Alfa SYF30RLIK
--Full time 2016 to 2019-- Seasonal now