Camera Script - Bird's Eye View

I'm trying to write a camera script which allows the player to view the scene from a bird's eye view as seen in most Real Time Strategy games. The zoom function that moves the camera along the Y axis was not much of a problem.

I'm having troubles with the rotation and movement. For the latter I used the mouse movement so that the camera moves along the positive X axis as soon as the mouse hits the right edge of the game window. Left, Forward and Backwards are done similarly.

 // Camera movement
 private void HandleMovement()
 {
     Vector3 mousePosition = Input.mousePosition;
     Vector3 movementDirection = Vector3.zero;

     // Left
     if (mousePosition.x <= leftBoundary)
     {
         movementDirection = Vector3.left * MovementSpeed;
     }
     // Right
     else if (mousePosition.x >= rightBoundary)
     {
         movementDirection = Vector3.right * MovementSpeed;
     }
     // Backwards
     if (mousePosition.y <= topBoundary)
     {
         movementDirection = Vector3.back * MovementSpeed;
     }
     // Forward
     else if (mousePosition.y >= bottomBoundary)
     {
         movementDirection = Vector3.forward * MovementSpeed;
     }

     transform.Translate(movementDirection * Time.deltaTime, Space.World);
 }

Due to the Space.World Parameter in Translate() the camera will always consider the global +Z axis as it's forward direction. This means the camera won't move forward into the direction it is facing when rotated around the Y axis but rather forward along the world's Z axis. Switching to Space.Self doesn't seem to be a solution since the camera should always move orthogonal to the Y axis.

I was hoping that somebody could help me finding a solution for the camera's rotation.

edit: Rotation Method & fixed title

 private void HandleRotation()
 {
     if (Input.GetMouseButton(2))
     {
         currentMousePosition = Input.mousePosition.x;

         float rotation = 0.0f;

         if (currentMousePosition < previousMousePosition)
         {
             rotation = -RotationSpeed * Time.deltaTime;
         }
         else if (currentMousePosition > previousMousePosition)
         {
             rotation = RotationSpeed * Time.deltaTime;
         }

         transform.RotateAround(Vector3.up, rotation);

         previousMousePosition = currentMousePosition;
     }
 }