Old building - authorities and planning costs

We want to buy the property from our neighbor. It is about 1200m² in size, fully developed, and there is an old farmhouse (built around the early 20th century) on the property with a footprint of 7x13m. The house has a living area of approximately 200m², spread over 3 floors including a converted attic. The purchase price for the house and land would be €25,000. It already has a new roof and a new electrical connection box has been installed. We actually wanted to convert this house into a residential building. However, for renewed long-term use (vacant since about 1988), the ground floor and first floor would have to be completely gutted. The ceiling between the ground floor and first floor would also have to be removed and reinstalled. After all demolition work, the house would be a skeleton, basically a shell for further construction. We would have to completely rebuild all units such as heating, sanitation, electricity, etc.

We know what the notary and real estate transfer tax cost, but which official procedures do we have to carry out, which applications do we have to submit, and which planning must be done before the work begins?

Best regards, René