The bedroom also immediately caught my attention. Try furnishing it. If you place the wardrobe on the left or right wall, there is barely half a meter of space on either side of the bed.
Since you want to be able to divide the whole thing into two living units later if needed, I assume you would then live downstairs in old age. That means you should already consider accessibility for this case. The guest room there really cannot be made into a senior-friendly bedroom (keyword: turning space for a walker or wheelchair...). I know, as a young, healthy person, you don't like to think that far ahead. But we experienced in our family how someone became a care case at 50...
For a very rough initial price estimate, add up the living area and multiply it by the average price per sqm determined for your area. It tends to be more expensive in the south than in the north, so you can probably calculate with €1500...
We also first tried designing our house ourselves. With moderate results. Honestly, the biggest investment of your life is worth using a professional. So, write down what you want and go to an architect! Oh yes, to get a feel for room sizes, prefab house exhibitions are a good thing. You can simply see how, for example, 12, 16, or 20 sqm children's rooms feel live and furnished.