Financing of 400k with 60-120k equity capital through a combination of BANK/KfW/savings contract

Yes, your landlord is not allowed to charge the maintenance reserve to your additional costs. Furthermore, some other additional costs are not allowed either. Those are then included in your rent. But you cannot do that yourself; you actually have the costs. That is why fixed additional costs are somewhat higher with ownership and especially with a house. Whether this is compensated by lower variable consumption costs depends on the individual case.

Let's briefly summarize:

You want to take on a maximum of 300' debt capital + 140' equity. Additionally, you can certainly estimate another 30' employee loan [EL]. This makes a total budget of 470' minus 70' land. Then 400' remain for the house + incidental construction costs.

Let's set incidental construction costs at 60'. That leaves 340' for the house. For that, you get approximately 130 m² living space on a slab foundation without much frills with a single garage.

For the incidental costs of the single-family house, you should estimate monthly 2.50 EUR to 3.00 EUR per m² - the bank will do the same. This results in incidental costs of 325 to 390 EUR per month.

Your additional costs will probably change. In addition to the classic costs such as electricity, heating and garbage fees, there are now also, for example, property tax, which must be paid regularly.

The actual additional costs will not change. Property tax is, for example, also a component of additional costs included in the rent.

In addition, the contributions to building and household insurance will probably be higher for a new building than for a "rather old half-timbered house."

Why? Household insurance has nothing to do with the age of the property, but with the household contents. Building insurance should be lower because insurers rate new buildings significantly better than old houses – especially if it is old half-timbering (= high risk due to flammable wood in the walls). Reserves must be accumulated. At least from the 10th year onwards. Before that, it is not worthwhile, and for no reason.

The actual additional costs will not change. Property tax is, for example, also a component of the additional costs in rent.

Why? Household contents have nothing to do with the age of the building, but with the household contents. The building insurance should be lower, as insurers rate new buildings significantly better than old houses – especially when it comes to old half-timbered houses (= high risk due to combustible wood in the walls).

Reserves must be built up. At least from the 10th year onward. Before that, it is not worthwhile, for what reason either.

I assume that the underlying standard value for the property tax in a new building is significantly higher than that of an old building.

Why, from my point of view, the household contents will be more expensive? I assume that in a new owner-occupied house with possibly more space, there will also be more stuff or higher-value items inside, which will accordingly be insured for a higher amount. Example: rental apartment: used kitchen with hardly any residual value, new building house: new kitchen for 15,000 euros. And this difference will be found in further furnishings and accordingly lead to a higher insured sum.

The earlier you start with the reserves, the lower your effective initial amount. Of course, you can also start after ten years, but I personally find that very bold.

In addition to the classic costs such as electricity, heating, and garbage fees, there are now also, for example, property taxes that must be paid regularly. Furthermore, the contributions to building and household insurance will probably be higher for a new building than for a "quite old half-timbered house."

All the mentioned costs are already paid now as a tenant/resident!
Of course, they can be higher, but energy costs are significantly lower in a new building.

I assume,

I assume,

I find

Many assumptions. But this also applies if, for example, you rent a different apartment (e.g., a larger one). However, it is generally not worth mentioning. The kitchen is also not separately assessed. Normally, the insurance should replace the new values and not the actual values.
And regarding the reserves. One can also save oneself to death. In a normal new building, usually nothing happens in the first 20 years. Then perhaps the heating system starts causing issues. But €5,000 for a new boiler/heat pump does not require 20 years of saving. Those who have enough money can certainly start early. To all normal home builders, I say that 10 years is enough. A "normal" reserve for all sorts of things is (or at least should be) already covered by 2-4 months' salary. This also covers repairs in the first 20 years.