tensioner every time with cambelt and the water pump is well worth doing, but while this is off get a new thermostat and housing, thermostats are a common weak point on the cooling system and when you replace it with a new seal the plastic elbow housing is a complete pain to get back on without cracking it!
the 4 allen bolts are the ones that hold the ribbed belt crank pulley to the cambelt pulley, they're very common for rounding off once being old and rusty so well worth swapping these for stainless versions