Financial planning for a condominium

Invest the money in funds or something else.
This is way too risky.
You obviously have no idea about the subject. A single rental apartment is a terrible concentration risk and may require patience that you don't have.
I also see the cost of living and the savings rate nicely calculated, that doesn't match the equity.

if you want to speculate and there are still 24 years until retirement. Bullet loan with a 25-year term without repayment. At the same time, saving money in ETFs or funds. A tax advisor should calculate it once

We are considering buying a condominium to rent out and live in later in old age:

Person 1: 43 years old, income 3500 EUR net on 13.5 salaries plus a bonus of 8000 euros annually, secondary income: 450 euros per month

Person 2: 36 years old, pension of 600 euros unlimited

Person 3: 5 years old, 200 euros child benefit

That sounds to me like a significant investment will be needed to close the supply gap into which Person 2 would fall when surviving Person 1.

Uiuiui - to quote Samson once

A particularly interesting property is a fixer-upper house of 76 sqm. The house costs 150,000 euros. Renovation will certainly be another 50,000. Unfortunately, we are not craftsmen [Scout ID 115061022 Pretty end-terrace house in the popular Duisburg district of Duissern]

Interesting for what? - a gibbet hill for daredevil properties?
The property seems to have had the reason to be abandoned, probably only partly the heavenly retirement apartment it was supposed to be. I know that attractiveness is relative and especially in big cities with social structural deficiencies there are worse holes, but as an investment property this will be a bucket with a perpetual hole, i.e. it will require a lot of investment first to be rentable with a tiny surplus, and at your age it will be ready for demolition despite all the effort put in now. "Pretty end-terrace house" is really an extremely quaint euphemism for that.

Interesting property...
... but not beautiful. Definitely western Duissern. Renovated Maybe refurbished, rentable to a single for €10. For couples, due to the large supply in Duisburg, already not easy to rent out. In the current condition rather for €450-500 cold, fitting for a house in that condition with low utility value (garden unusable as a garden).
Heating and windows are old - unlike an apartment building, all costs are on you. Garden is low-maintenance and easy to cover with artificial turf.
I’m also frequently dealing with real estate in the Ruhr area, but I wouldn’t take on this house.

Interesting object...

Interesting to tear it down and build a 3.5-story multi-family house with 3x 2-room apartments on it (contours of the neighboring house). It will only be difficult with the parking spaces ...