Now this is weird as hell. For about a week I saw a bit of cooland leaking from the expansion tank area. It was wet under. None of the hoses had any issues. I got the new one and installed it. Dang, my engine bay looks like a new car.

There were indeed a couple of tiny cracks under the old tank. So I put in 100% yellow coolant, took it for a spin, thought I noticed a wee bit of antifreeze smell still but way less, must just be some residue still burning off.

But whoops, I came home today, much liquid under the car and it had a horrible sweet taste. I see it's now dripping slow but steady off the oil pan just under the water pump. And the nicely translucent expansion tank shows that most of the new stuff drained out. Oops, probably not a good idea to limp with it several days on distilled water. Might have pushed the pump over the edge.

The photos I've seen of the water pump for this car are a tad intimidating. It almost looks like there's an external housing that remains and you're buying the replacement guts.

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1986 300SDL, 362K
1984 300D, 138K

Ah just watched a vid by a guy from Kent, WA (I think that's right) about the pump and I see the pump layout clearly now. Could be this is still the original pump - 132K at purchase. Like the guy in the vid says, it's a high possibility failure item, good part to renew.

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1986 300SDL, 362K
1984 300D, 138K

The pump isn't hard to replace, and not too expensive either. That does sound like where your failure is.
The pump bolts can get rusty and difficult to remove, penetrating oil ahead of time is always a good idea.

They are pretty tough. I replaced the original water pump on my '81 300SD a couple of years ago because I was hearing a screech. It turned out it was not the water pump at all but the alternator.

However the original water pump itself was pristine.
And it was not difficult to remove at all.

There are two types of impellers in these water pumps. Be sure you buy one with the cast impellers instead of the stamped out ones. That is the original design and is supposed to last much longer.

Good luck.

And although I watched horror stories about the bolts and needing to douse with LiquidWrench for days, mine came out easily.

(This was on an SD but I assume yours is the same.)

Use an anti-seize product on the bolts when you reinstall the pump.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

When My Water Pump leaked Coolant I found that one of the Bolts had been previously broken off. I opted to leave it alone and use sealant on both sides of the gasket and have had zero issues with it.

The attached pics show the new Water Pump made by Gates and the old Water Pump that had made in Japan on it. The Japan one has a sheet metal impeller and the Gats has a Cast one.
The Gates Pump runs ever so slightly hotter then the Japan one did.

When your advice turns out badly: On another Mercedes forum a person had broken off 2 of the Bolts holding the Water Pump on. I told him he could remover the Water Pump and the Aluminum mount behind the Water Pump and that would make it easier to drill out the broken bolts and at the same time allowed penetrating oil to be applied from the coolant side.
So he did that and more bolts broke but were the bolts holding the Aluminum mount behind the water pump.
I suspect he was rather angry at my advice.

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84 300D, 82 Volvo 244Gl Diesel