Could I make a basic map using a plane instead of a box??? Although then it probably wouldn't have any walls.

you have choices for making your walls/cliffs...
you can use the edge option and use shift and drag the edges or you can use the verticies and
move them then create polygons connecting the vertices to the ones you moved.

Plane modeling is mostly what I do. Yeah you can do that easily. Though just remember that you have to have a sealed world. If you want to seal it it's easy too really if you use editable poly and make sky from extruding edges at the ends of the plane. Then all you got to do is model the planes on top for the top of the sky. Just remember though tool does not accept editable poly when exporting a .jms so you would need to switch back to editable mesh before exporting.

If any of this confuses you, you can ask me more about it on xfire. My username there is the exact same as here. I am always (so long as I have time) willing to help other people with some basic BSP modeling techniques.