I thought they stopped putting them in these trucks in 87 but I could be wrong, my Dad has an 85 with one and it could tow anything ya want it was just a slug off the line. I am thinking about looking for one, so as long as it checks out good I would suggest it.
The 6.9 is a great engine. Plenty of power, and it's pretty economical too. Should get between 16 and 20 mpg.
The early 6.9's pre 85 had head gasket problems. Apparently from improper torque at the factory. In 1986 to 1988 the 6.9's had soft rings. This lead to premature wear and use of oil. check the tail pipe. If it's thick with soot you have an oil burner. Another way to check is to start it cold. If you get a cloud of blue the rings are likely bad. Rev it up after it warms up...if you get a blue cloud then it's burning oil.
If it is burning oil the engine will still run good if the fuel system is good. Diesels burn oil real well. You will get mileage off the oil, but it's expensive fuel at a buck a quart!
I have a 6.9 in my truck with 300,000 miles on it and it's still running strong. But it is using oil. Thanks to Ford using soft rings. I do hear that the properly rebuilt 6.9's were using better rings, so if the motor is a second motor with 85,000 on it it has a lot of life left. A diesel motor should run for 500,000 miles.
I had a 86 F 250 with the 6.9 and it was a great truck. If you run 3 inch exhaust, single pipe you will get 22 24 mpg. The only think i had go wrong was the water seperator? i dont no nuthin bout them things. I just routed around it with some fuel line.