Prefab house from 1976 - ready for renovation or demolition?

To me, this sounds like a project that is great to implement. I myself have renovated two houses that are older.

Basement: the rooms to be heated I would personally insulate professionally from the inside towards the outside. The basement ceiling can remain as is, since it is within the thermal envelope.
The other rooms should receive ceiling insulation. Use PUR or PIR for this, low build-up with good effect.
Equip the living rooms with wall heating.

Ground floor/upper floor: remove everything except plaster and screed. Check if milling in underfloor heating is possible in the screed. If yes, you gain comfort with relatively little work. I would have the installation plan carried out by a specialized engineering office (TGA planner).

I would budget around 25k for the electrical work. Include network and sufficient sockets right away. Also consider possible future electric roller shutters.

Bathrooms: think about water and wastewater. Two bathrooms can be quite expensive if you go all out. Also provide (have installed) wall heating in the bathrooms.

Windows: I have no reference.

Do the top floor ceiling yourself. Lay 2 x 200mm stone wool crosswise. The most difficult part is getting the rolls under the roof.

You can save especially with your own work on: structural work, gutting bathrooms and kitchen, basement ceiling insulation, basement interior insulation, top floor ceiling insulation, filling slots (electrical), pouring the underfloor heating and leveling the floor, small jobs after window replacement etc.

I would wait with the heating system. When everything is done, I would have the room-by-room heating load determined based on the available data and then see what results. With underfloor heating you are prepared for everything. Underfloor heating will cost only slightly more than new radiators. But the flow temperature can be significantly lowered.

With 200k you should manage with some own work. I would probably estimate (major items)
Electrical 25k
Windows 30k
Bathrooms 50k including water/wastewater
Insulation 5k
Underfloor heating including connection 10k
I don’t want to estimate the kitchen because it is not value-increasing and thus a consumer good

Many small things will still be added. But especially outside labor is expensive. Even though own work is often doubted here, I can say from my own experience that it can pay off financially. And these are not jobs for which you have to be an absolute professional. You have to be diligent and able to endure muscle soreness.

I always exclude kitchens. They have nothing to do with renovation. For the rest, 300k should easily be enough. You are far from a complete renovation. - 40k windows - 5k front door - 3k insulation of upper floor ceiling - 3k radiators - 30k heat pump with accessories - 30k floors - 30k painter - 30k new electrical system - 40k new bathrooms Then you can live well and still have money for other measures. These are arbitrary numbers that can fit. But they don’t have to – it depends on you and your wishes. Or on your own work. If conceivable, I would actually insulate the entire basement ceiling and try not to "live" in the basement.

Thank you very much for the assessment! Living in the basement is indeed not planned, but the offices are spatially necessary. What do you think of the solution in the comment below, to insulate the "inhabited" basement rooms not through the ceiling, but towards the exterior walls?

For me, this sounds like a project that's super easy to implement. I myself have renovated two houses that are older.

Basement: I would personally insulate the rooms that are to be heated properly from the inside towards the outside. The basement ceiling can remain as is since it’s inside the thermal envelope.
The other rooms should get ceiling insulation. Use PUR or PIR for this, low build-up with good effect.
Equip the living rooms with wall heating.

Ground floor/upper floor: strip everything out, except plaster and screed. Check the screed to see if it’s possible to mill in underfloor heating. If yes, you gain comfort with relatively little work. I would have the installation plan made by a specialized engineering office (TGA planner).

I would plan the electrical work with about 25k. Include network and enough sockets right away. Also think about potential future electric shutters.

Bathrooms: think about water and wastewater. Two bathrooms can get quite expensive if you go all out. Also plan for wall heating in the bathrooms.

Windows: I have no clue.

Do the top floor ceiling insulation yourself. Lay 2 x 200mm rock wool crosswise. The hardest part is getting the rolls up under the roof.

You can save especially with your own work on: main work, gutting bathrooms and kitchen, basement ceiling insulation, basement internal insulation, top floor ceiling insulation, filling chases (electrical), pouring in the underfloor heating and leveling the floor, small jobs after window replacement etc.

I would wait with the heating. When everything is done, I would have the heating load per room determined based on the existing data and then see what comes out. Because of the underfloor heating, you are prepared for everything. Underfloor heating costs only slightly more than new radiators. But the flow temperature can be lowered significantly.

With 200k you should get by with some own work. I would probably budget (major items)
Electrical 25k
Windows 30k
Bathrooms 50k including water/wastewater
Insulation 5k
Underfloor heating including connection 10k
I don’t want to estimate the kitchen as it doesn’t add value and is therefore a consumer good.

Many small things will come up as well. But especially outside work is expensive. Even if own work is often doubted here, I can say from my own experience that it can pay off financially. And these are not jobs for which you have to be an absolute professional. You have to be diligent and able to bear muscle soreness.

Thanks to you too for this assessment! I find the tip about the heating very good. Hopefully, the energy consultant will also make this suggestion; ideally, in this order, an oversized heat pump can be avoided.

What do you think of the solution in the comment below, to insulate the "inhabited" basement rooms not via the ceiling but towards the exterior walls?

Feasible if planned sensibly. But insulate the ceiling anyway.

Reasonably planned, feasible. But still insulate the ceiling.

Please explain this. Within a thermal envelope, rooms are generally not insulated against each other. After all, you also do not insulate the living room ceiling against the room above on the upper floor.

Please explain that. Within a thermal envelope, rooms are generally not insulated against each other. You also don’t insulate the living room ceiling against the room above on the upper floor.

Since I assume that a) the basement, when used as an office (and other heated or heatable rooms used as storage, etc.), is not regularly heated like genuine living space, and b) the floor slab is more or less uninsulated and will remain so, I would thermally separate these rooms in the basement. The thermal envelope at least leaks downward, and that becomes noticeable.