For that you need a Terrain-Shader and atleast the Commercial Licence.
GS offers its own Shader, which blends between your Textures according to your blendmap.
There are also some other Terrain-Shaders around this FOrum, try to search for it!
about your second question: as far as i know, you can only export a hightmap out of external software such as Terragen or as i using it EarthSculpur (ES) and import it into MED.
In ES you can also export the mesh with your desired vertices number, but that works only for models not terrain as its not a regular grid.