I had a 1991 190E 2.6. Bought it used from my best friend, who maintained it faithfully. I never had any coolant leaks, but the fan clutch went on it iirc.

Owning a 1988 300CE with the M103.983 Mercedes-Benz W124 - Wikipedia, the free encyclopedia for the past 12 years, I've had my share of coolant issues, from the radiator seam splitting in the center of the radiator to water pump failure to a leaking expansion tank.
The HG was replaced on mine when I bought it. Oil weeping out the rear of the block. No water leaking at that time. The W124 radiator neck was made of plastic at one time and was prone to splitting Mine never did. I don't know if this was fixed before 92. I did experience some leaking before the water pump failed.
A year later another small leak started before the radiator seam split was discovered.

So, it could be leaking from anywhere. The HG being the culprit would be down on my list a little. Shouldn't be hard to locate an external leak.

The 190e 6cyl shares a lot of parts with a 300e. My CE has a 190e fuel distributor in it that a dealer swapped out for testing.....12 years ago.

I have always used MB (red) coolant. Never anything else.