Here in the forum, construction is sometimes very expensive, which is not necessarily representative.
I would roughly calculate a total budget of half a million (house, land, kitchen, landscaping). But you can also do quite a bit by not building too elaborately and large and also doing some work yourself. Without children, it's much easier than with, and part-time work is also great for getting things done ;)
With the rate, I would expect about €1800, then it becomes reasonable. That is not little for your net income, but I find it okay for two people.
Existing properties I find difficult. You have already seen the issue with the additional costs yourself. In addition, many houses for 400k are simply in poor condition and you have to put quite a bit of money into bringing a 1970s house energetically into the 21st century. Bathrooms, etc. also tend to eat up money. Young used ones are rare and then are mostly expensive.
€1,800 was also our targeted rate, but given the current interest rate situation, I consider that unrealistic. You quickly reach €2,100 and more. Therefore, we hesitate. Regarding personal contribution: we would certainly provide that for the interior finishing but probably can’t factor it into the financing. Both of us have two left hands and unfortunately, we also don’t have a circle of friends with skilled craftsmen. Time is also a significant factor. I work in an area where 100% work for 50-70% pay is not uncommon. A scandal, but it can’t be changed.
Regarding existing properties, my assessment corresponds to yours. That’s why we have so far kept our distance from that option.