In solid construction, I can't imagine that you can get much cheaper thanks to the minimum wage and the many regulations in the construction sector. In the case of the prefabricated house, the house is probably manufactured in Poland. The wages there might be somewhat lower. As long as everything goes well, you can certainly get a bargain, but woe if something happens. Especially when you are supposed to pay a 30% down payment without receiving any service at all.
No matter which provider you choose, you are currently selecting your providers based on price, so you only have the cheapest providers included. However, all additional options are often very expensive with these providers. In my opinion, you are desperately trying to realize your dream of a house within your budget.
I hardly believe that you are aware of which additional costs come along with the house and property and what it means to finance a €350,000 loan with your income.