[*]take less space than a desktop
Only if you count the screen and keyboard, which you shouldn't if you're talking about hooking up a screen and keyboard. Laptops only have a size advantage if you like their screen and keyboard well enough to live with them.
There are desktops which are smaller than any laptop now.
[*]are built from the start to be very silent, as they can't be put under a desk
It's not true anymore. And you can build silent desktops.
[*]don't have room for a second hard drive, but I can connect a small NAS to my LAN to back up data
Some laptops take two or even three drives, not counting the ExpressCard slots which take SSDs, the memory card slots and so on.
[*]have good enough CPUs for moderate use
But the main advantage of laptops is of course portability. Even if they rarely leave your home, you can carry them from a room to another, from a desk to a couch and so in.
If not, what brands and models (available in Europe) would you recommend for a very silent and good quality laptop for moderate use (Internet, LibreOffice, small development in .Net and Python, etc.)?
It depends on your preferences regarding screen and keyboard (and therefore size). That's the most important part of a laptop and some builders are only good for some types of laptop. And I'm not knowledgable about some sizes.
Go out and try your acquaintances' laptops. Try the laptops shops have on display.