Gas heating

Good day,
my girlfriend bought a house where the first stone was laid in 1750. Some additions were made.
Ground floor, 1st floor and 2nd floor. (2nd floor would be the attic conversion that has not yet been completed.)
A gas boiler for 3 rooms is available.
The other rooms are equipped with stove heating. (A quite mixed range from all-burner to tiled stove.)
Now she considered purchasing a gas boiler for the entire house, as it cools down very much in winter and one is quite busy heating, and one is also not constantly at home.
Now the question arose, if one acquires a larger gas boiler, what should be considered?
The boiler will be increased from, I believe, 5-7 kW to about 24 kW.
Are the connections standardized?
Or is there anything to consider?
Should one use copper pipes or plastic?
What diameter should the pipes have?
Do you need a heat pump? And if yes, how is that calculated?
So as you can see, I am still completely at the beginning, and I thank you for any information to shed some light on my dark ignorance.
Thank you for the response,
and I wish everyone much success with their building :-)

just found something interesting.phew :confused::)
didn't see it earlier

uh the T in the headline obviously doesn't belong there :(

Hello,

so a central supply certainly makes sense with an existing mix, especially because the performance spectrum of the system can be better utilized in this way.

Regarding the other questions, it sounds very much as if you or your girlfriend want to do or carry out a lot yourselves.

Here I have a question which I consider not unimportant: is the house used only by your girlfriend or is part of it rented out? Because if something is rented out, I would avoid doing it yourself; it is much better to have a company that also assumes liability. Likewise, the insurance may not cover damages if only self-performed work was done without the expertise of a company.

As for the pipes themselves, I would tend to prefer plastic composite pipes, but that is partly also a matter of taste.

You will not need a heat pump, but depending on the type of system and the heating circuits as well as the pipe lengths, one or more additional water pumps may be required to ensure that the water is distributed accordingly.

But especially the liability issue, also with a gas connection, would give me pause and at least parts of the system should be carried out by a professional, and if possible, the entire system should be planned by a professional to avoid making an expensive investment that later possibly causes even more problems.

Best regards, Erik