Any engine will have issues with sitting idle too long. In my mind, it's more about how those miles were put on than the gross number itself. 5-6k miles per year is a fair amount of driving for a family rig that's being used for weekend trips, week-long vacations, etc... If a family has been driving the rig 5-6k per year, for the past 10 years, and they've been maintaining it along the way, that would be a reasonable rig to consider.
On the other hand, if they drive that 60k miles in the first -56 years, and then the kids graduated and it's been sitting in the driveway immobile for the last 4, that's an entirely different can of worms.
Understanding exactly how it's been used can help you make a much more informed decision about where the concerns may be.