I have the Davia system with two spal fans on my SL, fitted in 94 before the car was registered. Unlike the mb ac the spal fans only come on with the a/c
It's always been fine, and certainly doesn't cause overheating problems. Sometimes hot day, a/c traffic or long inclines the temp goes up to 90/100c, then settles back to 80, winter it never goes above 85.
Are you sure the correct thermostat is fitted?