I have an 86' 300E, a bit bigger engine but still the same 6 cylinder, milage is about 240.00k and as far as i know it is still the original engine and transmission (I am not the original owner),
engine burns about 2 quarts of oil per 3000 miles, transmision is fine. I change the oil every 3000 miles and the transmission oil and filter yearly. Maintenance, maintenance, maintenance. I live in Toronto, the winter is painfully cold the summer is hot. Car runs just fine. Power is down because of worn engine buit it starts every morning and runs just fine. Engine overhaul is not scheduled just jet. I think you got some time to go before any major trouble. My friend drives an 88' 300E with more miles then mine and his car is fine as well.