An older 50K mile diesel is hard to pass up. Chances are it's been babied.
I was talking to a PowerStroke guy that works at the NAPA store last week. His 6.0 has 155K miles, and he's had no problem at all. His thoughts are that most problems occur earlier, and if they make it to 150K, they may have an even longer life. Of course, he bought it cheap enough to where he can fix any problems that pop up and he's still ahead. The EGR problems were even worse 2005 and later.
I later was talking to a couple of great diesel mechanics. They've worked on so many 6.0's that they can easily correct the problem areas.