I haven't read all the answers, but my opinion on your draft:
Honestly, for €450,000 I don't find the floor plan very successful; everything would be too small and cramped for me. I think the main reason is that you started with an external dimension of 10x10, if I understand that correctly. Try it the other way around. First draw the rooms without the exterior wall. Then see how the rooms can be adjusted slightly to fit into an overall rectangular outer shell. Your building plot is big enough. That's how we did it, and our building plot only had room to maneuver by a few centimeters in each direction if we wanted to leave space for a garden and carport.
Regarding the doors, I would like to add that doors positioned directly next to an adjoining wall are always unfavorable. You will eventually need every wall for furniture. As you've placed the doors, 1. only small furniture fits, and 2. when entering the room, you immediately run into or look directly at the door. So, if possible, leave 60-70 cm of space from adjoining walls.
But don’t worry, we had like 100 different drafts. You’re at V1 – it will get there.
About the costs: building for €450,000 or more is no problem at all. But I would say it should also be possible with 150 sqm x €1,800 + €50,000 basement + €30,000 garage + €30,000 incidental building costs + €10,000 kitchen = €390,000. Without the architect’s fee definitely much cheaper, depending on the region, of course.