Is it possible to sell self-generated electricity to tenants of a multi-family building?

That is nonsense. The complete renovation cannot be passed on to the tenants. And if tenants could really buy electricity for 20 cents (,without basic fee), that would be a real bargain. Where else can you get electricity cheaper?

A: insulation only (10,000) and renewal of the gas boilers when they break down. Out of 4 units from 1994, 3 are still running. 1 was renewed last year. B: including insulation and heat pump minus subsidy: 75,000,--

I still don’t understand ... in option B the insulation should only be 10,000 euros, and then the heat pump minus subsidy should cost 65,000 euros? Is it made of gold? Or what other work is supposed to be covered by that?

I still don't understand ... in variant B the insulation should only be 10,000 euros, and then the heat pump after deduction of the subsidy is supposed to cost 65,000 euros? Is it made of gold? Or which other work is supposed to be covered there?

Since I don't know the circumstances of the house, I can only guess - but based on my assumption, 65k€ for the heat pump with everything and already after subsidy is not a problem.

Consider: You have to completely rework the entire heating concept! The thing currently has four completely autonomous apartments, each with its own heating and water supply; all of that has to be newly installed, connected, and above all centrally and cleanly planned.

That's something completely different from replacing a single gas heating system for all 4 apartments... and the stress during that time with the tenants... I'd go crazy...

Exactly. For example, the thermostats on all radiators must be replaced.

I think it is now also more understandable that I would still recommend at most A...

Such a conversion only makes sense if the house is eventually completely sold, all tenants are driven away, and then some large family acquires the place and renovates it.

That's when you can do something like that...

How much of the 70k is actually for the heat pump + new hydraulics? Because that seems very expensive for the size of the house.

There are also heat pumps designed to replace gas-powered individual heaters. Surely that is cheaper.

Then one could even consider switching to split air conditioning + instantaneous water heater. That should all be cheaper than this offer.

PV should be considered completely separately whether the investment makes sense or not. Being the electricity provider for the tenants probably not. Either as a full feed-in or simply leasing the roof, there are companies that do that on their own account.