Financing for new construction with land

I would like to have some tips from experienced financiers:

We are planning a new build. This should cost about 300,000 euros including the land and everything else. We are both civil servants, with my wife working part-time and me full-time. We have one child and together we earn about 2,950 euros net. This already deducts health insurance. We have 60,000 euros available as equity. In addition, there is a small reserve which should not be touched. So, 240,000 euros would need to be financed. The monthly installment for a pure bank loan would be about 620 euros at 1% repayment and a 10-year term and about 930 euros at 2% repayment and 15 years interest rate lock. I actually aim for the latter constellation, although it might be tight? After all, you want to go on vacation sometimes or there are repairs to be made or the purchase of a car, etc. I calculate about 350 euros per month as incidental costs for the house. The house is supposed to have a living area of 130 sqm and be operated with an air heat pump. Unfortunately, I have no experience with incidental costs or air heat pumps. With this type of heating, the monthly costs together with Kfw70 standard are supposed to be lower than, for example, with gas. What do you think? Thanks for your answers.

On the one hand, I already doubt the mentioned construction costs. Usually this and that gets added, and the costs increase by 30,000 or 50,000 euros. Furthermore, in your financing, you assume a repayment of 1 or 2%. After 10 or 15 years, not much would have been repaid and you might have to refinance at significantly higher interest rates. In your place, I would not want to finance 240,000 euros + amount X.

Regards