plastic thermo-housing/elbo isnt expensive. i tend to replace mine everytime i do a water pump. ie when i do a timing belt. its plastic, its smart.
id suggest the stock temp of 87C (thermo), as someone suggested a hotter replacement.
new coolant?
if not^ when i drop mine, i drop in a clean coolant pan. it gets road dirt and oil in it. i take a empty coolant jug and a funnel, take a bounty paper towel, double it over and use in funnel as a filter. make sure no openings betw towel and funnel on sides. filter works for me, as far as usable coolant, thats not too dirty from the pan. after check what you filtered/recovered, its 'clear' (its red of course), no oil film on top.