Wow, I've been Googling like mad trying to find a good W115-specific writeup for water pump replacement, but to no avail.
Does anyone have a good resource for such a thing? Mechanically it seems simple enough but I always like to have a guide if possible.

Thanks!

The engines of that date, regardless of which one is it, are all about the same when it comes to the water pump.

First, buy a new one. These are cheap.

On the actual repair.... Pull the radiator and oil cooler. This is not difficult but it is time consuming. You will need the room as it is tight in there.

Remove the engine fan. Loosen the belts that drive the water pump. Remove the pullies on the front of the pump. These sometimes take a slight tap with a large hammer. A bit of Liquid Wrench or equal will also help this to break loose.

You will then see that the water pump assembly comes in two parts: The water pump housing and the water pump. Since you will have the new pump in your hand you will be able to see where the bolts are. You will not need to remove the aluminum housing. The pump unbolts from this housing and and then removed from it.

This is another place where some penetrating oil will help. The water pump will be fused into the housing and will not want to come loose. But it will. Careful with the hammer here as now you are banging on the housing. It will crack.

When the water pump is removed then look inside the housing. If is it starting to show signs of being eaten up then this would be a good time to shop for a replacement housing. Also look for the presence of a thin wire wrapped up in there. If you find this it means your bottom radiator hose is coming apart.

Unless your upper and lower radiator hoses are new this would be a good time to replace them, anyway.

Then just put it all back together. Fill the system with the right coolant, run the engine and check for leaks. The upper radiator tank sits higher than the engine so there is no need to 'bleed' the cooling system as it will do this its' self over time. So watch the coolant level and add as necessary.

Anyway.... That's how I do this job. Anyone else is welcome to chip in if they have another tip I have missed.

I appreciate the tips, it should be very helpful in getting this car back on the road.

My time has been limited to fuss with the 300D, but it is finally back together. New water pump, thermostat, all belts, oil and fuel filters, trans filter, pan gasket and ATF. My job was complicated a bit by the A/C on my car, but it was inop for the time being so I didn't stress too much.

Having the car on jack stands made the oil canister removal easy. The water pump required removal of the AC condenser, radiator and shroud. That sure helped access, but getting the fan clutch off of the pulley was a minor strain. I discovered a leaky trans cooler hose, and replaced that while I was at it. The valve cover bolts were a bit loose also and contributing to my driveway drips.

After refilling fluids, I had an air bubble in the cooling system. Removing the hoses from the firewall valve assembly and giving everything a gentle flush and fill from the garden hose fixed that.

Pretty satisfying to have cured a couple of leaks and have an engine that actually reaches operating temp. Next up is a diesel purge. I'm excited to see how the stuff works on my car.