FYI, your car's engine is not prone to the head gasket woes as the M104 engines of '93+ models. you have the M103 engine which is not particularly prone to the malady. on the off chance that you do have the problem, you will find oil in the form of chocolate colored crud in your coolant reservoir. the test they did for exhaust gases in your coolant is not really relevant to MB engines.
the thermostat, coolant concentration etc. are all still relevant although i don't believe that the fan clutch is ever at fault most of the time. your fan clutches could very well have been ok. it seems that they activate when it is way too late in the game - but that's another story. since you are located in the "tropics" then your cooling system really has to be in optimum condition to keep your engine happy. in the meantime, you may want to investigate the "cooling harness" mod from one of our members. it will give you peace of mind, since your aux fans seem to help with the problem. btw, if you replace the thermosat. make sure you get the Behr thermostat as the concensus seems to be that they are much better than the Wahler ones.
and lastly about the overflow hose - there is at least one post that made a reference to it - never found out much about it...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"