Sculpting brush - low poly with normal map? I start again (it became a mess).

Simply put, I'd like to sculpt with a custom brush on this level.



Problem is, it will be too high poly. 

So what I'd like, is the following shape DECIMATED, with the normal map baked on it, and use that as a sculpting brush, and sculpt that details on the level in picture 1. 

Just like a texture, but low poly AND with normal map on it.



But I just can sculpt the texture, in high poly, or just paint with the normal map, in 2D.
I'd like both of the informations combined : lowpoly + its' normal map, in one sculpt brush.

Any other solution welcome.

Edit :

I followed your instructions, but it leaves me with a simple normal map that will sculpt high details. 

In fact, I want to sculpt low details (decimated) but have a superposition of normal map that will give the high detail, without poly counts. All of this in a sculpt brush. It seems it's not possible then.

(htt ps://i.stack.imgur.com/ExyMo.png)

(htt ps://i.stack.imgur.com/RGni7.png)
You got a very interesting problem to solve. You could bake normal map from high poly,sculpted chunk of terrain to a texture,and then use this texture as a brush in texture painting mode to paint onto a terrain texture and save it as a normal map for the terrain. 

To to that, You have to have new image texture active in material of a UV mapped plane. Place this plane above high poly terrain, turn on Selected to active in Bake options panel, select first high poly terrain, and then the plane and hit Bake - that's all you have to in general do. But here is more detailed tutorial on the topic