I would use a heightmap...
using this you will be able to create your terrain without shaping each part individually...
"In computer graphics, a heightmap or heightfield is a raster image used to store values, such as surface elevation data, for display in 3D computer graphics."
and use of the heightmap in Unity3D:
I hope this helps...you don't really need external 3D modelers for terrain, this helped me greatly..I hope you aswell