Since you have already covered on the Timing belt, I would not go into the detail. Now coming to the replacement of Crank Seal, the replacement is only done when there is a oil leak. The seepage starts around the main pulley towards the bottom of the engine. To replace it is not rocket science, Yes there are few obtacles that needs to be removed in order to have a decent space for the removal of the main pulley, All you need is a crankshaft pulleypuller. Now one thing is quite important to note while using this tool, the seal is rite behind the pulley once removed, so this has to be done quite slowly, by giving a few taps( wood mallet or fibre) around quite mild, so that you dont damage the surface and and then you slide the new one in and tap it evenly into place. The Cams replacement I have not heard of replacement even on high mileage, unless you have a total dead compression, My LS400 has reached 323000 miles and to date I have not done anything to it, and rest replacement as per the books have stuck to it, and still runs like a top.