Hi again,
Yes, a texture on any mesh (plane or cube) is a good idea. In the upper-right corner of your 3D viewport there is a ‘Maximize or Restore this Viewport’ button. Click that and then you can use middle-mouse + drag in the orthographic viewports to measure distance.
About your spiky imported mountains - Heightmaps are just gray-values (0-255?) so “reducing the color” in the 2d image might be an option. (Also try scaling down the landscape in Z-axis)
For background hills - consider just taking an image of it and masking out the sky.
Cheers!