Can someone help me add a wave to my Spawner script!!!

Can someone help me add a wave to my spawn script, each wave i would like it to have a certain time so it can go to the next wave and the speed can increase, i would like it to be editable in the inspector. so i can edit the time and speed. Also within my script their is a "Random.Range" function, but it is not that random, can someone help me to make it a bit more random.

Here's my spawn script:

      pragma strict

      // Variable to store the enemy prefab
       public var enemy : GameObject;

      // Variable to know how fast we should create new enemies
       public var spawnTime : float = 2;

        function Start() {  
        // Call the 'addEnemy' function every 'spawnTime' seconds
         InvokeRepeating("addEnemy", spawnTime, spawnTime);
         }

          // New function to spawn an enemy
          function addEnemy() {  
          // Variables to store the X position of the spawn object
          // See image below
           var x1 = transform.position.x - GetComponent.<Renderer>().bounds.size.x/2;
            var x2 = transform.position.x + GetComponent.<Renderer>().bounds.size.x/2;

             // Randomly pick a point within the spawn object
              var spawnPoint = new Vector2(Random.Range(x1, x2), transform.position.y);

              // Create an enemy at the 'spawnPoint' position
               Instantiate(enemy, spawnPoint, Quaternion.identity);
                }

Thank you!