It all depends what is your level.
It could be a 2d image, a text file, a 3d model, a serialized python object, a key-value table, or any combination of these.
It is very likely that doing all of it in your modeling software will be hard or you will need to process the data exported.
If you want a editor that can:
-paint uneven terrain (heightmap)
-paint textures on the terrain
-pain grass patches on the terrain
-paint and generate a nav mesh
-generate a collision mesh
-put objects on the terrain
-put actors on the terrain
-write tags on the objects
-write and read all of the above to common files (png, egg, json, csv)
then you can try my editor, the shaders for now are a bit bugged, but the data exported is 100% valid: