This is a tough call. After all, the 6.0 is the 6.0, and there are no guarantees with any vehicle with 180k + on the clock.
On the other hand, if you want a 7+ passenger vehicle that can tow heavy and is equipped with a diesel, you have exactly one option!
Me, if that is really what I needed, I would probably roll the dice and make the purchase -- sounds like you might have found one with as good a chance for success as you can. And when running right, the 6.0/5 spd torqshift is a good combination.
But keep that saving account handy just in case!