Purchase financing: how much equity (with the low interest rates)?

Nussbaum is right; pay as little as possible upfront, and stretch the repayment as long as possible. From 2021 onwards, debts will be "inflated away" again. You seem to know your way around the stock market, so stay in. There is no easier way to make money in the long term…

The salary looks like you could easily pay the repayment and interest. As a stock-interested person, you probably know that stocks (broadly diversified, e.g. in an ETF) over the last 150(?) years, even if bought at the worst possible time, have never resulted in losses longer than 13(?) years. So the worst case would be: both unemployed, and at the same time a stock crash, and selling the stocks at a bad time to be able to service the loan. I personally would only contribute the minimum amount of equity for which acceptable interest rates are available (10% + incidental costs?) and form another 10% as a cash reserve to be able to pay the installments in the worst-case scenario without having to sell the stocks at a bad time.

That's exactly how I would do it. And if you want a bit more diversification, I might contribute 20% equity + incidental costs. You are currently selling stocks at peak prices. At the same time, your 'retirement provision' portfolio remains solid. Then 10 years fixed interest at 0.65% and off you go.

Thanks for the input.

I think the hint about the cash reserve makes sense, especially when it comes to owning a house. I hadn’t really considered that.

That means out of the 300k, 26K equity, 17K additional costs, 60K to invest further, and 20K cash as a reserve. That leaves 175k to work with. Especially since the family will probably contribute something as well. I always play the poor sucker who's being drained by the family :D

I actually want to try investing in real estate as a capital investment. On paper, extremely charming.

Since you apparently prefer investing in the capital market over contributing equity, take a look at the loan conditions with a broker and use just as much equity as you need to get an excellent interest rate. Ultimately, there are several good ways, because besides optimizing costs and returns, there is also the component of feeling comfortable, which you slightly mentioned. If you make the economically best choice but cannot sleep well because of it, you have made a bad decision. In the end, most people buy a house for themselves and their family because it is associated with a good quality of life. Consistently focusing on quality of life instead of economic efficiency or any house features and imposed quasi-standards is something I consider highly sensible.

Since you apparently prefer investing in the capital market over bringing in equity, take a look at the loan conditions through a broker and use exactly as much equity as you need to get an excellent interest rate.
Ultimately, there are several good ways, because besides optimizing costs and returns, there is also the component of feeling comfortable that you quietly mentioned. If you make the economically best choice but cannot sleep well, you have made a bad decision. In the end, most people buy a house for themselves and their family because it is associated with a good quality of life. Consistently focusing on quality of life instead of profitability or some house features and quasi-imposed standards is, in my opinion, very sensible.

Wow, thank you.
I probably only addressed this component rather unconsciously.
But really engaging with it actively makes a lot of sense. In the end, one could say, for example, 100% debt with 2% repayment would probably be economically best, but with 80 to 90% and 3% I sleep better.
Really thanks again for the push to rethink the topic.

As many have already mentioned, as little equity as possible, as much equity as necessary to get a good interest rate from the bank.