X2 what mach4 said. I'd put the emphasis on the compression test and the marvel mystery oil. These old diesels can get carbon built up on the rings and in the grooves that prevents the rings from moving. On both of my cars when they were down for a little while, I filled the cylinders with MMO and let it seep down over the course of a week. I kept topping it off and letting it soak. Changing the oil drained all the MMO out of the crank case and then I spun the engine with the injectors out to blow any extra MMO out of the cylinders. It fixed a rough rocking idle on the 85.

The thing is, the engine wears out and tolerances start expanding. The expanding tolerances cause more wear and cause oil consumption. To deal with the oil consumption you use straight 50 molasses instead of 15W-40 oil. That thick oil is hard to pump, so it starves the engine of lubrication at start up and it starves it at high rpm (both times when wear occurrence is highest). That creates a nasty positive feedback loop and destroys the engine in short order.