Creating a level editor, question about GUI/interface

I'm creating a level editor using Unity3D and have a question about the GUI and interface for it. I'm looking to make my level editor look as closely to the Unity3D GUI and thinking of different ways of going about it. I would like to be able to edit and rotate like Unity3D does and the general "feel" you get using Unity3D. Also, if I do this in Unity3D, do I need to make it run like a game, or is there another "editor" mode that I can run in? What I want the users to be able to do is be able to add/edit/delete objects without giving them full control of them. Thanks in advance, -John