in general, if u take off the oil cap and its not all foamy and milkshake-like underneath it, ur headgasket should be ok. the coolant is gonna force its way into the oil bc its under high pressure. thats just in general though, it could still be bad. if its not all foamy underneath it then id go ahead with the thermostat and the temp sensor, since its apparently is prone to go bad. thats just my though. i dont wanna get blamed if ur engine blows lol