Heat pump in a 30-year-old multi-family house?

I am the owner of a house with 4 rented apartments (each about 75 sqm). Built in 1994, each apartment has its own gas boiler. At the beginning of the year, the first boiler broke down and I had to quickly install a new one because otherwise the tenant would not have had any hot water. Costs: €5,800. It is to be expected that the other 3 boilers will stop working one after the other. But it would be pointless to install gas boilers again now. An acquaintance suggested: heat pump and insulating the exterior wall with blown-in insulation. All this could be prepared without disturbing the tenants. The roof would also be suitable for photovoltaics – the roof is free of shade on the east and west sides. How would the billing work if the heat pump is temporarily "fed" by the photovoltaic system? My investment should not benefit the tenants financially only. Is there funding for rented properties?

Heat pumps instead of gas floor heaters would be possible, or also a central one. However, for billing and operation, individual systems are better. There are slim small devices suitable for replacing floor heaters. Unfortunately, installers usually charge absurd prices for this. A house from 1994 usually has a proper exterior wall. If there is still a ventilated cavity behind a brick facade, it must be checked on a case-by-case basis whether single-shell insulation makes sense. This can generally only be answered with yes for old buildings built before 1978.