Purchase decision small semi-detached house 100 sqm from 1930

Did you then install the heat pump in a basement room, or outside? Outside would be rather difficult for us ...
And can you give an example of such a radiator? Or is that not allowed here in the forum? :)
Can I find your project here so I can take a look?

Yes, sure, underfloor heating would be great ... just because of the space savings alone.
But I'm afraid that will be difficult for us. Does anyone have experience with ceiling heating? Is it as simple as I imagine, meaning installation on the ceiling and covering with drywall in the form of a suspended ceiling?

There are also underfloor heating systems that have a very low height. [Deckenheizungen] are not widespread without reason. Heat rises upwards.

Below is screed and above wooden beam ceiling(s) - I need to get comprehensive advice on what would be possible. Everything naturally depends on the heating ... and heat pumps also currently have corresponding delivery times.

Did you install the heat pump in a basement room or outside? Outside would be rather difficult for us...

No, classic outdoor installation. It will be difficult for you in the basement, because the heat pump needs endless outside air to draw its energy from. I only know of one heat pump that was installed indoors. Then with large air ducts (approximately 70x70 cm).

And can you give an example of such a radiator?

33 radiators. You can find them anywhere – they are normal radiators, but "thicker."
Or (these are expensive) low-temperature radiators specially designed for heat pumps. These are radiators that also have a small fan installed (like a PC fan). You can quickly find them on Google as well.

There are also underfloor heating systems that have a very low height.

That is, of course, also an alternative. In a thin-layer system, you only need a few centimeters in case of doubt. Not advertising, just an example: Schlüter Bekotec Therm.

Below is screed and above wooden beam ceilings - I need to get comprehensive advice on what would be possible.
Of course, everything depends on the heating ... and heat pumps currently also have corresponding delivery times.

You will need that time for the renovation anyway.
I think your plan is overall feasible. But this is not something you do in a few weeks.
Last year we also gutted our house. Completely, because it was going to be demolished. We needed about 3 weeks for that, but I was at home and worked full days every day, my husband worked daily after work and on weekends, my older son about 100 hours, a friend a few days, and my younger son also 1.5 days. It is easy to underestimate this, but it is physically hard work. If you’re not used to it, you quickly reach your limits. The same goes for the rebuild. Just choosing and procuring the materials means moving a lot of weight. Usually, you store it temporarily and then install it. Most things you move 3 to 4 times.

Regarding the wall you want to tear down, I would also consult a structural engineer. Maybe it’s not even a load-bearing wall?

I don’t recommend ceiling heating because warm air rises. I would rather forego suspending the ceiling and install underfloor heating.

I would prefer to forgo hanging the ceilings and install underfloor heating.

That is exactly what I would also favor. But before anyone starts work anywhere, please evaluate and plan all renovation positions with a professional (engineering office has already been mentioned). What is the current clear room height? Heat pump possibly on the garage roof?