is this a good deal

im thinking of buying a 95 nissan maxima.the guys says its a temp sensor that is making it run ok one minute and want to die the next.thats what he was told,but i found 3 temp sensors on autozone an ambient air, a coolant and an egr temp.which one of these would it be and how would i replace it.this is all going to be considered in what hes asking.hes asking 800 but its got 200,000 on it.im thinking 600.any ideas would be appreciated