I used just one single layer for the whole scene and Blender handled it very well. Also, the models are very simple, the trees for example has about 35k polygons each and for the hills with forests in the distance I used alpha planes.
The clouds are hand painted.
I tried combed particles, but it wasn’t working as I espected, so I used the tangent factor in the velocity pannel to get that look.
Everything except the sky is Blender
For the water I used a volumetric shader and fake caustics. The underwater geometry helped a lot too.
Thanks for all the other comments guys!