You need to get a head gasket, some valves (obviously) and a rocker cover gasket to be on the safe side. Are you sure you broke the thermostat? Was it round and looked something like a valve? This opens and closes to regulare the water flowing around the engine. You can get new ones from halfords or most motor spares shops and they're not expensive. You'll need a new gasket for that as well, one for the top (the bit that seals the head) and one for the bottom, (the bit that seals to be block).
Whilst you've got the head off check for scoring of the cylinders, and 'blow-back' this is where the hot gasses escape through the gaps that have been scored in the cylinder, if this is occuring there will be dark scorch marks on the side of the cylinders, it should be obvious if its been happening for a while.
And when you put the whole thing back together you'll need a torque wrench to make sure the head bolts are at the correct torque. The head bolts go on in the reverse order in which the came out, so start from the middle and work out.
Whilst you've got the head off check for scoring of the cylinders, and 'blow-back' this is where the hot gasses escape through the gaps that have been scored in the cylinder, if this is occuring there will be dark scorch marks on the side of the cylinders, it should be obvious if its been happening for a while.
And when you put the whole thing back together you'll need a torque wrench to make sure the head bolts are at the correct torque. The head bolts go on in the reverse order in which the came out, so start from the middle and work out.