Property purchased - start construction planning before handover?

The question remains unanswered. Then sign the contract first.

If the plot is flat, then you don't need a survey at first. Is the plot in the [Geoportal]? Then you can get the survey points from the surveying office for a fee.

We have actually found a nice plot free of developers in the Berlin surrounding area

Congratulations!

The ground will be sand, it is a moraine. There is no swamp or rock in the area, only sand.

Is the sand load-bearing? I would ask if the building ground survey can already be done. That means, first playing around a bit with the dimensions, orientation, house ideas, concepts, and floor plan. After that, a preferred position of the house usually emerges. Have a soil survey done, architect or general contractor, whom you, as you say, have already scoped out, create a plan in case a prefab house doesn’t fit. So in summary: engage with the future house and building partner.

The question remains unanswered. So first sign the contract.

Which question? Sorry, I must have overlooked that.
The contract is signed.

The tip about the Geoportal was good. In the BrandenburgViewer you can also see the soil levels of the surrounding fields (other side of the street): sand.
We can also read measurements there, but we are not quite sure because of the flatness. The front part appears to be level, this is where the house is supposed to be built (at the same height as number 9), the back part is wooded and slopes down in two steps about 1.5m lower.

(The picture is just photographed, the marked area is the stretch between the fence and the tree that is to remain.)

How long is the street frontage? I would rather position the house like No.5. Otherwise, you have the neighbor exactly to the south.

What is that at house No.5, it looks so messy?

The street frontage is 17.5 m, and it goes back almost 50 m. After 19 m, the property drops 1.5 m, where two steps and a staircase were built. And the area is very wooded. So you would have to fell a lot (if you get permission) and properly shape the terrain. We really like the little forest at the back; there is currently a hut and a nice shady and hidden terrace.

I have no idea what is going on at No. 5; when we viewed it, there were normal beds and it looked neat. No. 9 is completely being renovated.