Do the water pump and have the coolant system flushed, have the head skimmed or replaced if you think its cracked. Remeber to replace the coolant with antifreeze mix.
The headgasket itself normally doesnt fail, its normally the head that fails because of a fault somewhere else (water pump/thermostat). Even though the thermostat was changed for a new one i would do a test on it in a saucepan of water to check it opens.
Do the cambet/tensioner and change the plugs at the same time just because you take them off. If you buy a gasket set you will get all the gaskets you need but just check that they are all in the kit just in case.