Modernize a house with night storage heaters, what are the options?

- Replace hot water boiler with hot water heat pump (basement model without external unit)
- Night storage heaters remain for now, but if possible not used or only in individual rooms
- Air heat pump air conditioning in the following rooms: 2x children's rooms, 1x living room, 1x bedroom, possibly 1x office
- Pellet stove in living room with ventilation to hallway and kitchen
- Photovoltaic system with storage

Sounds like a good plan to me. DHW heat pump, as already mentioned, probably requires outside air intake.

170 m² from the 1970s, if nothing has been modernized, will probably be around 200-250 kWh/m² with a total heating load of 17 kW – as a rough guideline.
Insulating the top floor ceiling and replacing the windows would be sensible short-term measures.
Also upgrading the roof energetically, possibly requiring a re-roofing.

If unrenovated so far, you will probably end up with around 10,000-12,000 kWh of electricity with the heat pump, minus the pellet amounts.
What does the energy certificate say? There should be an energy demand certificate for the house. If it is any good, it also lists sensible modernization measures.

We once shortlisted an old house that had [Nachtspeicheröfen] installed. During the viewing, we had an energy consultant with us. He advised us back then to use a combination of heat pump and pellets. This was suggested because the house had a large, mostly dry cellar suitable for pellets. He did not completely advise against a pure heat pump, but it depends on how well the house can be insulated otherwise. Otherwise, it can quickly become expensive. At the time, we considered renovating it up to the 85 standard. However, even in the preliminary planning, it was so expensive that we ultimately did not dare to proceed. We were afraid the project would be a "bottomless pit."

Since you do not provide any information to substantiate your details (pictures, a scout ID, or similar - caution, no external links!), I have my doubts about the combination "storage heaters / 170 sqm / year of construction 1976" (and by the way also skepticism regarding your list of measures; you would be underestimating or sugar-coating the total modernization effort).

Year of construction information often refers, especially with realtors or second owners, more to a modernization year. Storage heaters and 1976 are not mutually exclusive by themselves, but for new buildings of this size the combination would at least be unusual. I do not see anything in your list about windows and doors, piping and sanitary objects, namely of a basement, but not about the insulation of its ceiling. For example, would you be aware that installing underfloor heating can have consequences for the lintel heights of doors?

Please excuse that I could not send everything so quickly:

Attached is the energy certificate. Unfortunately, the exposé has been removed. So it is clear that in the coming years the wooden insulated glass windows will also be replaced, but not as a primary measure, as we want to do everything gradually.