People do barn finds all of the time. Heck I just pulled out an 83 Eldorado that has sat for 18 years.
Just remember condensation, corrosion and drying seals, will be your enemy in the early ownership timeframe. Oil floats and the oil pickup is at the bottom of the pan, heck you might even have algae in the diesel.
Somehow hearing the story of the old owner, I'm guessing things will just go fine. Last thing he wants is his trusty old horse to go to it's grave. Would do his heart good to see it being enjoyed by a family locally.