The 6.9 IDI is a very reliable engine, partly because it does not make enough power to break anything. The main problems with the 6.9 IDI are glow plugs after a certain mileage and injection pumps that can fail from lack of lubrication. The solution is to rewire the glow plugs to a manual button and replace the injection pump with an aftermarket unit. An aftermarket coolant filter to remove the water is also advised. Other than that the 6.9 IDI is a very reliable engine and will always get the job done. If the rest of the truck is in good shape, go for it.