Realistic financing opportunity?

Regarding additional construction costs:
Connection fees are included.
On the other hand, I have read from you about plumbing, which is not included.
So definitely calculate carefully.
And with laminate installation, also include the costs that are not visible. Glue, moldings, work equipment... about one-third is added as a flat rate to a flooring cost of €30/sqm.

Eigentümergemeinschaft und damit verbundenen gemeinsame Verwaltung kommt uns entgegen - gerade weil wir ja nicht einen (Lebens-)Traum vom Haus verwirklichen wollen, ist es schön bestimmte Aufgaben einfach abgeben zu können.

You do not delegate, you have to be present at the meetings to cast your vote. Then there is complaining about the poor street lighting and suddenly everyone has to pay their contribution for new LEDs.
Just a colleague’s anecdote.
But there is certainly worse. If necessary, you just sell again earlier. That’s the way things go.
Good luck.

Do you know how high the monthly costs for the property management will be for the [Eigentümergemeinschaft]?

I saved 30 euros out of 40 euros pocket money as an elementary school student. 75% savings rate! I wonder how you wasted your money during that time. I would have saved 130,500 euros in the same period! I would also be very interested to know how you did that. I guess a plot twist like "I lived rent-free in my parents' apartment." Or "my husband earned 8000 net. He covered the main expenses."

I don’t understand what the problem is supposed to be. 290€ rent + Hartz IV rate 432€ to live on = 722€ I still had 180€ more. And of course, that was, for example, a backpacking vacation through Thailand and not wellness trips... I don’t know how many students have more than 900€ available and we just continued the student lifestyle until the kids. But everyone has to decide for themselves what they spend their money on and how much they want to subordinate to the house. We were perfectly fine with it. To the OP: I would still choose the smaller house. The houses really look nice and also extremely affordable. I now understand the thing with the mother-in-law and find it understandable. I myself would find such a dependency on the husband extremely uncomfortable, but you have to decide that for yourself. If it somehow worked that your future income would be accepted by the bank and so that you would be there instead of the mother-in-law, it would surely be better for you.

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If you want to move to Nauen (or anywhere further out), you have to compare the local rents for nice apartments with the monthly installment. Someone already mentioned that here..

Well... I wouldn't move to Nauen for a nice rental apartment. It's not that bad, but I don't like it in Berlin that much. Then I'd rather try to keep our small apartment and expand it with a small garden plot. (Also not easy, but doable). Only in the combination of building wealth through home ownership, school enrollment, and no neighbors above us is Nauen interesting for us.

Regarding ancillary building costs:
Connection fees are included.
On the other hand, I have read from you about sanitary installations, which are not included.
So definitely calculate very carefully.
And when laying laminate, also include the costs you don't see. Glue, moldings, tools... about one third flat-rate added to a flooring of 30€/sqm.

You don't hand over the voting right, you have to be present at the meetings to cast your vote. Then there’s grumbling about poor street lighting and suddenly everyone has to pay their share for new LEDs.
Just an anecdote from a colleague.
But there are certainly worse things. If necessary, you just sell earlier again. That's the way things go.
Good luck.

Yes, that's possible. We'll manage the meeting and voting right... basically the parent-teacher meeting for the little house. For example, you don't have to take a half day off to have the heating system maintained in the basement, winter service is commissioned, and you don't have to get quotes from the craftsmen yourself... so quite a bit is taken care of or prepared in advance for decision-making.

Do you know how high the monthly costs for property management will be in the owners' association?

According to the example calculator from the developer, including management, waste disposal, etc. <350€ per month. However, still without reserves... I expect another 200-300€.

I try to take all objections seriously and they are justified.

you have to be present at the meetings,

No, you don't have to be present. You can also grant a proxy or simply choose not to cast your vote.

Well... I wouldn’t move to Nauen just for a nice rental apartment.

I can only advise you to maybe walk through an existing row house settlement now in the summer (also in the evening sometimes). Ultimately, a row house is just an apartment in a vertical format. We moved from the city to Pankow and, no joke, in the summer it was noticeably quieter in the apartment than it is now, not to mention the lack of anonymity. The balcony doors in the apartment were open all day long; in the row house, I often close the terrace door because the source of the noise is right next door. You just need to see where your own bedroom window is and where the neighbor’s terrace is—if someone with a high sociability factor lives there, you’re in trouble. In our settlement, there are one or two houses I would move out of immediately.
The only advantage over a condominium is certainly that there is some greenery attached and that the legal separation (apart from WEG street area—and yes, we also had expensive lighting fixture replacements that I didn’t need, but we also have correspondingly built reserves) is better with a house than with an apartment (issue of liability WEG etc.). But for that, you sometimes have to walk, as we do (basement to attic), over four floors with the laundry to the children’s room.
And although we made a huge economic lucky find (bought cheaply in a sought-after location, then the boom started) and our neighbors are nice people, the realization remains that I would never buy a row house again in my life.

Then I’d rather try to keep our small apartment and expand it with a small garden allotment.

This model exists twice in my environment—it’s surprisingly two families with very high household incomes. And if the rents in Berlin (Pankow) weren’t outrageously expensive, I think I would do the same. In the summer, you have a large garden with significantly more privacy than in a row house. In autumn, you just close it up until spring and have your peace.

Just because the income “only” suffices for a row house doesn’t mean you have to buy it. Be honest with yourselves...

Is Nauen really the desired spot? — Really good schools, but also far from work.
Is a towel-sized garden enough for me? — It’s enough for me, I don’t want gardening work, but I like sitting in other people’s “real” gardens.
Do I want to commit myself financially to such an extent? — The project is affordable, but probably can only be sold at a loss in the coming years...

Overall a rather difficult mix of factors, but no one said it would be easy. I’m keeping my fingers crossed that you make a decision you will still be convinced of in twenty years, whatever it may be.