It was not uncommon in the old days to get a million miles out of one before a rebuild.
On of the issues with former military trucks is the time they sit doing nothing. Seals dry out, pistons can get stuck, injector pumps go bad.
Ya pays your money and ya takes your chances.
The 250 is an obsolete engine, when I had my money pit (M816 wrecker) I could not find an old school mechanic to work on it.
Some of the 900 series 5 tons coming out now have the 8.3 turbo Cummins in them.
I love the one in my M927 but it is somewhat anemic in the big truck.