The heads, go without question, do them all and do them right or you are headed for another failure. The weak point in the head, and block is where the center cylinder meets up with the two outer cylinders, as there is a lack of water jacket between them. The engineers are relying on convection to keep it reasonably cool but when there is an overheat, those valves will be the victims for sure. The valves, especially the exhaust, rely on contact with the seat to cool by convection, in this case, that valve, whether it was a weak spring, guide galling, or gross mal-adjustment, stuck open, causing hot gases to constantly blow by, thus overheating the valve, thus burning it (Like Roy said) then causing the tell-tale 45deg notch break.
Depending on what your low end looks like. From the pics, it is quite impressive that you still have crosshatching on the piston cylinders after that many miles/kms. If there was an overheating situation, then there would be quite a bit of crap floating in the oil (however microscopically) so the bearings, rods and mains, could have some wear or galling. You won't know unless you look. If it looks good and clearances are in range than you could get by without. But........