The 6.9 is a DINOSAUR as you know, but aside from general maintenance to my knowledge there isn't a big blow up piece. The only major issue (and I don't remember if it was the 6.9 or 7.3 IDI, but it was one of the IDI's) had some cast issues in the blocks so some of them would crack. If it's got higher mileage you probably won't have that issue.. On a truck that old you're looking to make sure all your rubber lines are good, hoses aren't cracked, wires aren't exposed, etc. Basically just general wear items.

One big thing since this truck isn't a driver is that tells you that the grease in your fittings probably dried up / is no good. Go to all your U-joints (there will be 2 on the drive shaft on either end, and each end of the axle) and see if it has a zerk fitting. If it does, throw some grease in there. Same thing with tie rods and ball joints, basically spend some time under the truck and grease everything with a zerk. Also with any used diesel check your glow plugs and fuel injectors, those are your biggest failure points.