Is financing home construction and land acquisition realistic?

Good day everyone,
my partner and I (both 27 years old and civil servants at the federal level in the upper service) want to build a house in the Rhineland in NRW in the near future. For this purpose, we now have a plot of land in view. It costs an estimated €331,000 including ancillary purchase costs (614 square meters). The plot is located very precisely between Cologne and Bonn. The standard land value is €480.

According to rough calculations and efforts using several budget calculators, we come to a maximum budget of about €741,000.
Financial situation:
Household income (net): €5900
Equity capital: €249,000 (self-saved and support from parents)
Loan amount: €492,000

Consequently, about €409,000 would be left for the construction of the (prefabricated) house. However, this amount would have to include all purchase and construction incidental costs. Do you consider this possible?

Thank you in advance for your answers!

Consequently, approximately €409,000 would still be available for the construction of the (prefabricated) house.

If you deduct a flat rate of €50,000 for incidental construction costs, €20,000 for the kitchen, and €39,000 for outdoor facilities and other expenses, €300,000 remain. With this amount, you can get 100sqm or, with a lot of personal effort, a bit more.

If you deduct a flat rate of €50,000 for ancillary building costs, €20,000 for the kitchen, and €39,000 for outdoor facilities and other expenses, €300,000 remains.
For that, you get 100 sqm or with a lot of your own work, a little more.

Or you do the outdoor facilities yourself gradually, use your existing kitchen or a good used one or one from Ikea, and you can easily build 20 sqm more.

Thank you very much for your answers. That sounds quite positive at first. Does such financing sound solid to you? Or should there be any problems with the bank?

Or you do the outdoor facilities yourself step by step, take your previous kitchen or a good used one or one from Ikea and can easily build 20 sqm more.

Yes, I will definitely do the outdoor facilities myself. First of all, the house should be standing so that you can live inside. Outdoors as well as the garage will be done step by step.