thermostat/aux. fan relay

I have an '85 300SD -- live in the mid-Atlantic (some warm temps). The running temp is around 95-100C, sometimes goes over 100 but drops back at highway speed. Two questions: (1) What is the proper thermostat? I'm thinking of putting in an 80 degree T-stat, and would expect the running temp to be, say, 85-90? Does this seem right? (2) Where is the relay for the auxiliary fan? The fan is not coming on automatically (tested the fan itself, and it operates). I can't replace the temp switch on the A/C drier myself, but I think I can replace the relay -- if I can just find it. Thanks in advance.

Curt