Leave out the basement, build smaller, and look for a plot of land that is flat. Slopes are always more expensive. I think that with your equity it is quite possible to manage with a loan of €250,000 - €280,000. but that means a lot of compromises, and maybe also sacrifices. (A wood-burning stove doesn’t bring much, but is relatively expensive)
Take the Epis list, that usually fits, get advice on how much you can stretch, and then look for a suitable plot and a house you can afford on it. Or depending on the area, something used...
At the beginning, you seriously underestimate the additional costs. Our house cost €230,000 with all the bells and whistles, without a basement and additional costs. With the land and the mentioned additional costs, the price ends up starting with a 4. When we are finished with the outdoor facilities and fully furnished, the house price has simply doubled.