The Bull gear in the engine would be of concern to me with that kind of mileage. True, many Detroits go a long ways without a problem, but if that bull gear goes, you might as well consider an inframe or full rebuild. You will really chew up that engine. It is recommended by the OEM to replace the bull gear at 800,000 miles. About a $1400 job. And as good measure, replacing the crankshaft damper at the same time would be a good idea. Many that specialize in performance diesel engines are now recommending the damper be replaced at 500,000 miles. Yes, they do wear out, and there is no way you would know it until damage occurs. Even Detroit now is recommending the same thing for all of their engines. As was mentioned, a dyno run would give you a good idea about the condition of the engine. I would add that having an oil sample analysis run would be the icing on the cake. Paying for an hour of two of a good mechanic's time to go thru the truck with a fine tooth comb would be money well spent.
If it was taken well care of, and proper maintenance done when it was needed, there is no reason that truck should not have a lot of useful life left in it. And the price is not bad.