The water tank is a HSK500 (picture with German description) tank, which has a heat reservoir of 443 liters, and an internal heat exchanger for the domestic hot water (DHW) of 45 liters.
Why did I buy that one?
Hard to say...
There were others which had a bit more sophisticated pipe connections, better system of "layer loading", "hard foam" isolation, but also more expensive.
I also thought about buying one made of stainless steel, but I guess as I really fill it up to 95 degrees C, it will be full of lime after 20 years, so I will have to get a new one anyway. And they are a hell of a lot more expensive.
With the tank there came a 10 cm "soft foam" isolation that fulfills the minimum insulation requirements. But the cellar was a lot hotter than before (23 or 24°), and the tank cooled out in about 4 bad-weather days. Not bad, but it was clear that it could be better.
So after several months, I added another round of 16 cm aluminum coated glass wool, and now the tank stores enough heat for about 7 or 8 days when it is full loaded (temperature range is 95°C down to 40°C).The cellar room now looks like this. It does not look very nice or professional, but the best I could do in an evening. I don't care, and it works as expected.