My feed back from personal experience with a 6.4, from a guy who is a Ford diehard and has 6 different ford powerstroke superduty trucks:
I would be most leary of a 6.4 that has not been tuned and deleted that has some miles on it. Especially if it has high engine hours to miles driven ratio.
If it has been tuned and deleted from low hours and mileage, I think the risk is greatly reduced.
I have found out the hard way that an emissions compliant 6.4 will NOT tolerate short distance driving and extended periods of idling.
My 6.4 has been a problem child for me. My issues developed while it was 100% stock at about 40,000 miles and with higher then average engine hours.
Had I of known what I have learned from the whole fiasco, I would have tuned and deleted it from the day it left the dealer lot new. The emissions systems will cause those motors to self destruct in the type of use I typically do.
I know of many others who have 6.4s that drive extended lengths on emissions compliant 100% stock motors and have had no issues, well past the 150,000 miles and more mark.
Once I tuned and deleted mine it turned into an awesome monster. 50% mileage increase driving empty. 75-100% mileage increase towing heavy and unbelievable power and torque.
If you look seriously into a 6.4, I would check it out extremely thoroughly.
I know this experience does not strictly apply to the Ford 6.4 motors. The others will have similar results in similar driving conditions.
The risk in my opinion would be:
7.3 lowest risk
6.0 middle
6.4 most risk