Is this the most efficient way to attack the closest enemy?

BLUF: Is the C# code listed below an efficient way to attack the closest enemy?

Hello all,

I am an experienced level designer and took the plunge into unity about five days ago. I work full time so have only had about 20 hours of messing around with scripts in unity. My experience with programming prior to this was very minimal. I could look at basic code and more or less understand what was trying to be accomplished. I am currently going to be working on dozens of small projects to understand how to accomplish core fundamentals and basic tasks that most games contain.

The current one I am working on is basically this:

I have a turret in the center of an open field

Enemies (zombies... generic, I know) spawn at 4 points (north, south, east, and west) in 3 second intervals

The enemies move at a slow speed towards the turret

Once they are within range, the turret damages the closest enemy (that is within range).

I left a lot of variables public so I can monitor their status in the inspector while the scene plays.. so ignore that.

Please review this bit of code and tell me if there is a more efficient way to attack the closest enemy within range. It works as intended and I haven't noticed any issues with it.

 using UnityEngine;
 using System.Collections;
 
 public class Attack_Enemy : MonoBehaviour
 {
     public Vector3 playerPosition;                // Default 0, 0, 0.  Turret is in center of scene, value not needed.
     public Vector3 distanceDifference;            // Difference between the target and turret.
     public float oldDistance = Mathf.Infinity;    // Starts at infinity to always ensure IF loop runs to detect closest enemy.
     public float currentDistance;                // The sqrMagnitude of the difference in distance
     public float attackSpeed;                    // The rate of attack of the turret
     public float timer;                            // A timer that constantly runs
     public int attackDamage;                    // The amount of damage the turret does
     public GameObject enemy;                    // Current closest enemy
     public GameObject closest;                    // Assigned as closest target
     public GameObject[] targets;                // Array to load every target as they enter trigger
     public bool enemyInRange;                    // True/False if the enemy is within the trigger or not
     HealthZombie HealthZombie;                    // To adjust health of zombie from the HealthZombie script
 
     void OnTriggerEnter (Collider other)
     {
         if (other.gameObject.tag == "Enemy")
         {
             enemyInRange = true;
             targets = GameObject.FindGameObjectsWithTag ("Enemy");
             foreach (GameObject target in targets)
             {
                 distanceDifference = target.transform.position - playerPosition;
                 currentDistance = distanceDifference.sqrMagnitude;
                 if (currentDistance < oldDistance)
                 {
                     closest = target;
                     enemy = closest;
                     HealthZombie = enemy.GetComponent <HealthZombie> ();
                     oldDistance = currentDistance;
                 }
             }
             oldDistance = Mathf.Infinity;
         }
     }
 
     void OnTriggerExit (Collider other)
     {
         if (other.gameObject.tag == "Enemy")
         {
             enemyInRange = false;
         }
     }
 
     void Update()
     {
         timer += Time.deltaTime;
         if (timer >= attackSpeed && enemyInRange && HealthZombie.currentHealth > 0)
         {
             Attack ();
         }
     }
 
     void Attack ()
     {
         timer = 0f;
         HealthZombie.TakeDamage (attackDamage);
     }
 }