area selection (mouse drag) on overhead game

I have a single unit selection script that a friend and myself made and neither of us know how to make it into area drag, had a bit of a look and is proving a tad more challenging than anticipated. The camera is orthographic with an overhead view, using 3D models on a flat plane.

 using UnityEngine;
 using System.Collections;
 
 public class Select : MonoBehaviour {
     private GameObject lastHitObj;
     private GameObject selectedObj;
     public LayerMask placementLayerMask;
     public LayerMask ground;
     private RaycastHit hit;
     private RaycastHit pos;
     public Material mouseOver;
     public Material mouseExit;
     public float speed = 5.0f;
     private Vector3 moveTo;
     private bool moveing = true;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if(selectedObj){
             MoveObject();
         }
         var ray = Camera.main.ScreenPointToRay (Input.mousePosition); //create ray on mouse pos
         if(Physics.Raycast (ray,out hit,1000,placementLayerMask)){
             if(lastHitObj){
                 if(Input.GetMouseButtonDown(0)){
                     lastHitObj.renderer.material.color = Color.white;
                     selectedObj = lastHitObj.transform.parent.gameObject;
                 }
             }
             lastHitObj = hit.collider.gameObject;
             //mouseExit = lastHitObj.renderer.material;
             lastHitObj.renderer.material.color = Color.green;//set materal to hover mat
         }
         else{ //not on building block
             if(lastHitObj){// if previously hit something
                 lastHitObj.renderer.material.color = Color.white;
                 lastHitObj = null;
             }    
         }
     }
     
     void MoveObject(){
     
         var ray = Camera.main.ScreenPointToRay (Input.mousePosition); //create ray on mouse pos
             if(Physics.Raycast (ray,out pos,1000,ground)){
                 if(Input.GetMouseButtonDown(1)){
                 Debug.Log ("Running");
                 moveTo = pos.point;
                 if(moveing){
                     StartCoroutine (Moveing());
                     moveing = false;
                 }
                 
             }
         }
     }
     IEnumerator Moveing(){
         while(selectedObj.transform.position != moveTo){
             
             selectedObj.transform.position = Vector3.MoveTowards(selectedObj.transform.position, moveTo, speed * Time.deltaTime);
             selectedObj.transform.LookAt( new Vector3 (moveTo.x,moveTo.y,moveTo.z));
             yield return new WaitForSeconds(0.002f);
         }
         moveing = true;
     }
 }