Moisture / mold in the house - call an expert?

If the mold spots are the only problem, I would buy first. Minor renovations will be necessary anyway, and this will be dealt with then. For built year 64, there is probably no exterior wall insulation yet. When retrofitting it, this problem will be solved as well. Get a moisture meter (capacitive), it will provide (also later during inspections) indications. Otherwise, we already have a dryer in the basement (100€ at the hardware store, hygrometer-controlled), because in summer the moist outside air cools down there and could condense locally. In winter, this is not necessary anyway.

Otherwise, we have a dryer in the basement anyway (100€ at the hardware store, hygrometer-controlled), because in summer the humid outside air cools down there and could condense at certain points.

How about closing the window?

How about closing the window?

It works differently here:

The fresh air is first drawn across the basement through the central ventilation system.
In summer, it is pre-cooled this way; in winter, it takes up the waste heat from the heating
before going through the exhaust air heat exchanger into the living spaces.

With constantly closed basement windows, it would soon start to smell musty in the basement.

In any case, consult an expert …

First of all, thank you very much for your numerous responses! I really appreciate it! Today at noon I have an appointment at the house where the moisture measurements will be carried out & a contractor will assess the exterior waterproofing. If this is necessary, he will tell me the cost directly. Then it depends on whether the seller is willing to reduce the price a bit. I will report later.

Then it depends on whether the seller lets the price be pushed down a bit.

What makes you believe that this is necessary? Perhaps the seller has already deducted the known defects? Of course, we cannot say anything about this here because we know almost nothing about the house, its location, the asking price, etc., but immediately calling for a discount just because the house is not free of defects – which is absolutely to be expected given the year of construction – strikes me as an inappropriate reflex. Just because the house seems "too expensive" to you does not mean that it actually is. If you take a little time to familiarize yourself with the current situation on the real estate market, you will understand what I mean.