Possible funding amount / limit

Others have an overdraft of 10,000 EUR with that income. Nobody would care about that either.

A used overdraft of 10,000 euros wouldn't be an issue? (Sorry for staying off-topic ...)

A used overdraft of 10,000 euros wouldn’t be an issue? (Sorry for staying off-topic ...)

yes, it is included in the SCHUFA, so indirectly. we have an active line of credit of 10k which we don’t use at all. no one cared.

A used overdraft of 10,000 euros wouldn't be an issue? (Sorry for staying off-topic ...)

An overdraft of 10,000 euros used to the amount of 7,000 euros is not an issue. Rather, a positive sign that a) a bank has this trust and b) one is acting within the terms of their agreements and c) there may be reasons for it, like Christmas bonus next week or year-end bonus.... Who knows? And I find BAföG even more harmless. That is the highest asset of an investment.

yes, it goes into the SCHUFA, so indirectly. we have an active revolving credit line of 10k that we do not use at all. Nobody cared.

You cannot see the use of an overdraft in the Schufa. Also, not all banks report credit lines of this kind.

Then the gold credit card with a 10k EUR limit must be devil's work.

There is something going on here
But why has no one yet dealt with the numbers?

Income is at 3120+380€ = 3500€. Do not consider parental allowance since it disappears again after 12 months and is therefore irrelevant.
Of that, 1700€ are to be spent on the loan, which is almost 50% of the regular income currently available. That’s a big number! After deducting other liabilities (BAföG) of ~100€, 1700€ remain for additional costs and living expenses. Depending on the bank, this could already be tight for four people due to the household budget/liquidity. Of course, your own household budget must be correct, but if the bank already shows the red card there, that would be an important signal to me... go to Interhyp or Dr. Klein. If you find banks that go along with it, question it anyway for yourselves. Look over the advisor’s shoulder to see how many banks show red and how many green (you can see this nicely in their software). If the advisor says "yes, it works," but 100 show red and 3 green, that would still make me think.
I currently have a calculation from Ing-Diba here (who, by the way, are a joke when it comes to the household budget: ~920€ for the first adult, 200€ for each additional adult, and children don’t matter to them at all (apparently they cost nothing in Holland)), who always assume 3% repayment in the household budget.
Applied to the 1700€ per month, that means a loan amount of 408K€ at assumed 2% interest, i.e. 5% annuity.
The 2% is probably unrealistic for the desired 20-year fixed interest and 100% financing. Moreover, not 408K€ but 470K€ are desired, and something still has to be deducted from the monthly rate for the private loan to finance the incidental purchase costs. Plus all that has been forgotten so far (furniture, kitchen, renovation).

I don’t even bother to calculate it further. In my opinion, it’s a pipe dream like hasn’t been seen in the forum for a long time.

2) Our limit is probably max. 415,000 EUR (internet calculator). Our wish is 470,000 EUR (without incidental costs).

That sentence says it all. The limit in the internet calculators is pure madness and you want 15% more on top plus a private loan.

But why has no one dedicated themselves to the numbers yet?

Well, because ... oh, you wrote that yourself . Therefore, a quick reminder of the basics: pay off debts, build capital. A single-family house is just expensive.