ᐅ How do you get started? When should you hire which professionals for which tasks?

Hello,
I have decided to build a single-family house. I have already read a lot about it and could answer many detailed questions, but I can never find information about the first steps. How do I get an approximate idea of the cost of my desired house without immediately paying several thousand for an architect? How do I find a plot of land? How can I tell if my idea for the house will fit on the plot, or how many square meters (square feet) of living space would be suitable for me?

I know what I want:
A basement, underfloor heating, a heat pump with a cooling function (probably with a ground collector), photovoltaic panels and solar thermal system (possibly integrated directly into the roof without tiles underneath?), a smart home system (probably Homematic), a double garage with electric vehicle charging capabilities, and preparations for a home battery.
I also already know the type of rooms I want, the kind of features they should have, and roughly where they should be located in the house.
I assume I will need an architect to realize my wishes. But my questions are: When should I bring the architect on board? When do costs start to occur? And who else do I need? The construction companies will surely need to be coordinated by someone.

Regarding my financial possibilities, I still plan to discuss this with a banker friend, but at the moment, I estimate a budget of about 300,000 euros (approximately $330,000), with about 30% saved as equity so far.

I am currently single but want to plan the house so that it can comfortably accommodate up to four people with minimal renovations.