Finance buying land or rather leave it?

If the location of the property suits you, I would recommend you to go for it and buy the property. If necessary, you can always sell it again. The worst that can happen is that you have to sell at a slight loss in the end. But that can also happen if you invest your money in ETFs or similar and need it exactly when the next crisis is approaching us.

If one of you contributes significantly more equity, I would also make sure that person gets a larger share of the property. This way, in case of a separation (which of course I don’t wish for you), there will be no trouble.

Regarding the question of whether one should already deal with the topic of building at 24, I can't say much, but I find it still quite early.

The idea behind securing the plot now is:

This way you have more time to pay it off

And we assume that no new building area will be developed in this location again, and if so, then at even higher prices.
But of course, things could also develop quite differently and we might regret later that we bought at way too high a price with way too high interest rates. On the other hand, interest rates and prices could rise even higher and we did everything right.

I have always assumed that it is better to think about it and take action as early as possible.

That’s why I would pay down as much of the plot as possible now. When you are on the home stretch, you can start thinking about the kitchen and stuff.

I think it is most sensible for us to keep the rate at this level but always fully utilize the special repayments. That way, we have the "security" of not having a problem with the installment amount even in case of parental leave and still pay off a good amount.

Is my assumption correct that in this case the first special repayment should best be made directly after the start of the financing since it has the greatest impact then?

Clear: The earlier, the more special repayments, the better, because then you have to pay significantly less interest. I would therefore secure the property as quickly as possible (especially if it is in a place where there are not many building plots available every year) and pay it off very quickly. And I hardly believe that building land will ever become cheaper (keyword: anti-single-family house policy of a political party), because this is no longer politically desired by some parts. It is better to strike now. And in 5 years, construction costs/interest rates may also not be quite as high anymore.

I think it makes the most sense for us to keep the rate at this level, but always fully utilize the special repayments.

It is meant differently. Now you are (variably) only financing the land. Only when the house is specified will a new larger loan with new conditions come. And until then, repay as much as possible. 5% special repayment on the land I consider too low.

I also think you can’t go wrong initially with a piece of land. Sealing of surfaces with the same housing shortage is the current issue. I would also set the rate as high as possible. Maybe the parents can provide short-term support during [Elternzeit]?

Is there actually already building permit law for the property?
So is there a development plan for it?