From when is a heat pump worthwhile in an old building?

You don't get the idea to not heat several rooms at the same time (you don't do that either), you don't need a buffer storage tank. That saves thousands right at installation and just as much over the lifetime.

well, sometimes it's enough to heat only the ground floor and not the upper floor. Saves a lot and the temperature upstairs is still sufficient.

Yes... no... maybe. Probably not. A bit colder is okay. You shouldn't turn it off completely. On the one hand, the heat from the ground floor moves into the upper floor (through the ceiling). This leads to discomfort on the ground floor, the flow temperature has to be increased, and the savings effect is (possibly more than) gone. And if you don’t separate the two floors somewhat airtight, it can quickly lead to mold problems. (Warm, moist air rises and condenses in cold room corners) If you partially close the heating circuits upstairs and keep it at 17° C, then it works, the heat pump doesn’t have flow problems, and everything is fine again.