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Hi,The repairs that you are requiring are mostly circumstantial.The head gasket failure particularly on the diesel engines, especially the 2.5, is common. The rest of the repairs are going to be related to the circumstances of the failure.It is most probable that you had either a clogged radiator or a leak in the radiator that allowed the engine to over heat to the point of failing the head gasket. This accounts for the radiator and head gasket replacement.The thermostat and radiator cap are both standard procedure to replace after an overheat. This is because these are both thermal and pressure sensitive devices (the thermostat spring collapsing as the metal gets hotter, the radiator cap spring collapsing under pressure. When these devices are run too hot, the springs fatigue and their previously calibrated operation will be shot. It is typical for an overheated thermostat to never open again. This is why those items are being replaced.The lower sump seal is not all that typical, however if the head gasket failed in such a way that it allowed the oil and coolant to intermix, this would result in water/coolant in the sump and the only way to be sure to get all of it out would be to remove the pan and clean it and then clean the bottom end. This is good practice if fluid mixing occurs, as the coolant will turn highly acidic if left in the oil even in small amounts (and you could easily have 1/4 liter or so that simply would not drain out due to the shape of the sump.So the head gasket failure... again a known weak point and they will fail if run too hot on these trucks. The radiator could very likely be the reason the gasket failed, though it is possible that it over heated for another reason causing the radiator to crack.The thermostat and radiator cap are standard replacement during this type of repair as they rarely make it through an over heat condition, and the sump seal is so that they can do a proper job in cleaning everything up to make sure you don't have any future unexpected issues.