""Yes, inside we are totally flexible and we can still change the walls and rooms however you want.""
Have you already received a construction specification? It should state whether and how often the floor plan can be changed free of charge.
I can describe to you my approach from last year:
I composed an email in which I summarized all the conditions regarding approximate square meters, roof, windows, type of heating, number of bathrooms, and much more, together with a rough desired floor plan and a price range (I might be able to send you a template by private mail) with a request for an offer and the forwarding of the service description to the companies that might be an option from a model home park. Excluded were the companies I knew to be too expensive for me. I also mentioned that the plot is already owned and requested information on approximately how long the construction would take from the issuance of the building permit.
When I then received the first information in writing, I made appointments at the model home parks. On two days, I reserved a two-hour time slot for each company that seemed interesting to me.
Some companies did not respond to my email and were naturally excluded. Others did not want to provide a service description and were also ruled out; another company told me on the phone that they basically do not build without a ventilation system; another company said they basically do not build with a gas condensing boiler; or that the house (in our case a prefabricated house) could not be erected earlier than 2022 due to their order situation, and so on.