Common problematic areas w/ high mileage are front suspension, steering gear, rear differential, transmission. Any of those will fail from poor maintenance or just plain high mileage.
I’d have to disagree there, I’ve had a handful of 5.9s and put many miles on them without any engine troubles.And if going with a magnum motor try and go with the 5.2 (318) rather then with the 5.9 (360). I have had many friends who have had both and every one with the 5.9 had heads go out on them as well as many other issues....guys with the 5.2 were dead nuts reliable.