The engine does not have a level editor. However, since you already using Blender, you may want to check out BlenderPanda.
GLSL is the best support shader language in Panda at this moment. There is Cg support, but Cg is not really supported by the industry anymore, so I would not invest much time there. Panda probably has HLSL support, but I have not heard anyone mention it. Also, Panda’s OpenGL rendering is quite a bit ahead of its Direct3D rendering.
Performance of “complex” games is hard to speak to, but the engine has been used for large games.
Blender is probably one of the better supported editors at the moment.
For online resources, take a look at this manual page. The two books from Fireclaw the Fox are frequently updated and free. The manual is also a good place to start.