Which heat pump is needed for an old building?

Insulating the top floor ceiling as a DIY project should cost (to reach new build standards) an afternoon of work and under 1,000€.
THAT is one of the most effective measures you can do at all.

So the following conclusion: a practically uninsulated old building is energetically not renovated and only the windows are replaced.

That’s true, I could do that as a DIY project as well. That would also be an option.

If you look at it that way regarding energetic renovation, then windows, front door, and new underfloor heating will be milled and the new heating (heat pump).

energetic renovation concerns then window, front door

Only that changes the energy consumption of the house.

and new underfloor heating is milled and the new heating (heat pump).

The house does not care whether you bring your heating load into the house via heat pump with underfloor heating or make a campfire in every room. In the end, the house always consumes the same amount of energy. Therefore, the first measure is to reduce the energy consumption of the house. As far as I'm concerned, grab two buddies, three heavy sledgehammers, and simply remove the old screed. Then as much insulation as possible on the floor slab (in the upper floor it doesn't matter) and lay new screed. It has various advantages: - probably cheaper than milling - improve insulation = lower heating costs - underfloor heating can be laid individually. If otherwise nothing is done, I would almost tend to choose a 5 cm installation spacing everywhere (thus massively increasing the heating surface so that the supply temperature can be as low as possible) – that is not possible when milling

As far as I'm concerned, grab two mates, three heavy-duty sledgehammers, and just remove the old screed. Then put as much insulation as possible on the floor slab (it doesn’t matter in the upper floor) and lay new screed. Has various advantages:
- probably cheaper than milling

So just completely remove the screed on the ground floor and mill on the upper and attic floors?

Then install new insulation on the floor of the ground floor and probably also edge insulation if the new screed is to be installed.
New insulation and new screed plus the removal and disposal of the old screed—wouldn’t that be more expensive than just milling?

If it significantly improves things to add new insulation, then of course I have to consider it; if it’s only a bit more expensive, then I’ll see.

so only completely remove the screed on the ground floor and mill on the upper floor and attic?

Then apply new insulation on the floor on the ground floor and probably edge insulation as well if the new screed is to come. New insulation and new screed plus removing the old screed with disposal, isn't that more expensive together than just milling?

If it makes a significant difference to install new insulation, then of course, I have to see if it's only a bit more expensive.

You naturally also have to see what build-up heights you have available. I have just finished this on 120 sqm – disposal cost was €0, because I took it myself with my own trailer to the recycling center. You have to check what a container costs. New screed will probably be between €2.5k and €3k + new insulation. You can calculate it yourself – including the upper floor.

Now the question remains whether it should be a 7.5 kW heat pump or a 10.5 kW one, both offered by Vaillant.

Now the question remains whether it should be a 7.5 kW heat pump or a 10.5 kW, both offered by Vaillant.

A heat pump is always selected to be exactly the right size or slightly smaller than necessary. Please do not choose a model that is too large. The 10.5 kW variant would be out for me.