70s existing property, unrenovated, as a rental object

To be honest, it sounds to me as if there could be a lot of nasty surprises lurking, and you really don’t know what it will end up costing you.
Somewhere water is leaking into the ceiling... Aha... Where it has already caused damage is probably unknown, and might only become apparent when you start tearing it open and see that you have to keep going to find the end of the problem.
Could you easily afford something like that? If not, I wouldn’t do it.

Yes, through Myrna’s explanations, that’s basically settled now.
The water damage is probably still manageable. It only occurred when water was running in the upper floor, and that particular upper floor was unoccupied for a long time. Accordingly, no damage is visible in the adjacent rooms.
But you are of course right – you never really know.

In the event of a change of ownership, gas or oil heating boilers that are older than 30 years must be decommissioned and heating and hot water pipes in unheated rooms must be insulated.

That is generally incorrect. Renewal is only necessary if they are NOT condensing or low-temperature systems. Pipe insulation is correct.

[*]In addition, renovation obligations for the top floor ceiling arise from the Building Energy Act. If thermal insulation is missing there so far, the ceiling must be subsequently insulated, although exceptions are permitted.
[*]If not even the minimum thermal insulation is present, the Building Energy Act requires subsequent insulation. And this must then exceed the minimum thermal insulation.

This is also not correct. The roof or top floor ceiling must be insulated if there is no minimum thermal insulation. For insulation, the U-value must not exceed 0.24 W/m²K.
Quote: "However, there are exceptions: if the minimum thermal protection according to DIN 4108-2 of the top floor ceiling in an old building is met, the buyer does not have to insulate anew. Whether the house complies with this DIN regulation should best be checked by an expert before buying, recommends Thomas W, building advisor from the Association of Private Homeowners (Verband Privater Bauherren). The DIN standard is met if the house has an R-value – that is the value of the thermal resistance – of 0.90 watts per square meter and degree Kelvin. 'But that only applies to components that weigh more than 300 kilograms per square meter. And that weight is basically only reached by reinforced concrete,' explains the expert. For other ceilings, according to DIN 4108-2, an R-value of 1.57 applies, which corresponds to about seven centimeters of thicker mineral wool."

Therefore, it can be the case that the measures – for which one has two years – only cost a few hundred euros. On the top floor ceiling (i.e., above the upper floor apartment), for example, 24 cm of mineral wool would be sufficient as an insulation measure.

I would invest money in an expert who takes a look at the house and assesses the problem areas. I wouldn’t necessarily take on a major renovation now, but if everything else is alright and it only needs a small amount of money... why not? Of course, this also assumes that the bank would grant you an additional loan.

When single-family or two-family houses change owners, this often results in obligations for energy-efficient renovation for the new owners.

I am aware that this is required nowadays when renting out. Kati, if you see your life as a slave to a rented property, then go for it. If you finally want to find peace, let it be.

Do you have an emotional connection to the house because of the family history? Or is it about possibly getting a bargain? As an investment, two residential units would be too much of a concentration risk for me.

I am aware that this is required nowadays for rental.

The obligations are stated above. But it has nothing to do with rental, it also applies to personal use.