Best way to access game objects

I am currently working on a 2d game in which there are 2 players that have an "army" each. I want to detect when a unit of an army is in attack range of one of the enemy's unit. So I need to iterate through all the enemy units and select one that is in range.

What is the best way to do that ? There are a couple of methods that I think are a little slow for my taste and I would like to avoid. Like GameObject.Find() and GetComponent()..

I tried to create 2 empty game objects RedTeam and BlueTeam and added all units as children of these, but I can't seem to find a way to easily access them. I used getComponentsInChildren, but this seems to return the component in the parent too and I'm kind of scared that this method is "eating up" too much.

     Transform getNextUnitInRange ()
     {
         Transform[] enemyUnitsTransforms = otherTeam.GetComponentsInChildren<Transform> ();
         foreach (Transform enemyTransform in enemyUnitsTransforms) {
             if (Mathf.Abs (transform.position.x - enemyTransform.position.x) <= range) {
                 Debug.Log(transform.position.x + " " +enemyTransform.position.x);
                 return enemyTransform;
             }
         }
         return null;
     }

Should I make a list of somekind with the units and if so where should I store it so it's easily accessible ?

This is my first game in Unity, but I worked with other 2d game engines.