How much equity do you have? A paid, fully developed plot (according to BORIS NRW €57,500 - purchase price was €67,000), also: €40,000 savings - €15,000 kitchen, €7,500 furniture, €10,000 landscaping, €7,500 as a buffer. The plan here is to also continuously contribute from the current salary during the construction phase for materials, etc.
We are currently in the middle of the construction phase and our plot also "only" cost €67,500 for almost 400 sqm, and you have 650 sqm.
So I assume that building in your region is still not as explosively expensive as in some other regions. And in my opinion, you have many trades in the DIY work that already make a difference. Whether all of that works out with the DIY work only you can assess.
You can finance the whole thing with the figures given. In addition, you still receive payments that you have not factored in. Most people don’t go on trips or skip vacations during house construction anyway and put that money into the house (yes, I went away, but that was during the summer break with the general contractor and the vacation money was already saved ;) ) and afterward, you are usually busy with "moving in" for a while.
To me, the whole thing already seems well thought out. I can’t specify our expenses down to the exact 100 euros either, and yet my own expense planning was very accurate.
We also handle it the same way, paying some upcoming expenses from our current salary.
I would definitely increase the costs for the landscaping, even if it is done later, or take out more money. If anything is left over, it is easier to pay back than to raise money afterward.
And as soon as there is a child, you can also adjust the repayment rate for the time when the income decreases a bit. We did that back then too. We reduced the repayment to the minimum and, once the child was out of daycare and I worked and earned more again, we increased the repayment to the maximum rate.