Terrain you can make with various terrain programs that can export to heightmaps, like EarthSculptor or Terragen. You can load the heightmap into Panda3D using the HeightfieldTesselator class. You can find various examples on the forums here.
What excactly do you mean?
The best way to create your world is using plain python code, not using the Scene Editor. I never used it in my games.
Great. Is this a question?
Custom GUI? check the DirectGUI part here at the manual on the website. It’s under Documentation > Manual.