The temp gauge should read about 85 degrees C; so 65-70 is to cold. Thermostats are available in different temps; I don't know what they will cost in US $, but can't be that much.
The climate controls with temp wheels for both sides of the car were used in almost all the MB types in Europe (for heater-only systems as well) and tend to break down once in a while...(electronic failure)
Start with the thermostat and see what it will bring you..
with a minor 600+k