radiators...

[UtwigMU]

Go to library/amazon and borrow/order Architect's data by Neufert, should be available at least in English and in German (book exists and is updated since at least 1920s or 1930s).

It has all measurements and calculations and other neat info on such as how much radiators, windows you need, how much objects measure, ...

[VJ]

In a turn of events, it was chosen to simple renovate the outside of the radiators (sandpaper the paint off and repaint). There are several reasons why I decided to cancel the whole radiator exchange:
1. I never really wanted to get rid of the old ones; I think they look great, but everybody kept suggesting - almost pushing me - to change or fully restore them. And they work well.
2. The price... it also matters: the new radiators were expensive, about the price of a new kitchen for 5 radiators - due to the high pressure option.
3. The risks: they wanted to freeze the circuit to remove the radiators, while the central heating is switched on AND the circuit has a high pressure. It really scared me, especially when you combine it with the fact that I have an original old parquet floor. As it seemed they would bypass the administration (do it without approval or informing anybody), I'm quite sure no insurance would cover it if there were to be problems.

I managed to finally convince some friends of mine (where I am staying) that this might be the better option. But seeing the price offer for the 5 radiators, also immediately made them realize it is not worth it all. So they were also very eager to admit that it has no sense to change them.

UtwigMU: thanks for the suggestions, that might be useful in many situations.