Reuse Scenes just changing some variables??

I assume the number of enemies is a variable stored somewhere in a script in the scene?
Why not have a static script, or persistent GameObject, that is told how many enemies to spawn before loading the level, and when you load your scene, pull the relevant information from it?
For example:

 static class LevelData
 {
     public static int m_NumEnemies = 0;
     public static float m_Speed = 0;
 }

Example difficulty selection:

 if (difficulty == Difficulty.Easy)
 {
     LevelData.m_NumEnemies = 5;
     LevelData.m_Speed = 2.5f;
 }
 Application.LoadLevel("NextLevel");

Example player class:

 class Player : MonoBehaviour
 {
     float speed = 0;
 
     void Awake()
     {
         speed = LevelData.m_Speed;
     }
 }