Using Buttons to create units only when selecting the barracks.

Hi i need a little help here...

basically i need to click on a object like the (barracks)

then have it change the buttons on the hud to what ever i place them inside to be..

for example button 1 will spawn a sniper button 2 will spawn a rocketguy etc

at the moment i have my barracks spawning the two units by pressing keys on the keyboard..

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Buildings : MonoBehaviour
 {
     //Buttons for Hud
 
     public Rigidbody BuildQPrefab1, BuildQPrefab2;
     public Transform spawn;
 
     //drag Selection
     public bool selected = false;
     //private Vector3 moveToDest = Vector3.zero;
     public float floorOffSet = 1;
     //Click Select
     private bool selectionByClick = false;
     
     
     // Update is called once per frame
     private void Update () 
     {        
         HudButtonsUsage ();
 
         //Place Layer Unit Control Here.
         if (Input.GetMouseButton (0)) 
         {
             if (!selectionByClick)
             {
                 Vector3 camPos = Camera.main.WorldToScreenPoint (transform.position);
                 camPos.y = CamOP.InvertMouseY(camPos.y);
                 selected = CamOP.selection.Contains (camPos);
             }
             if (selected)
                 gameObject.GetComponentInChildren<Projector>().enabled = true;
             else
                 gameObject.GetComponentInChildren<Projector>().enabled = false;
         }
 
     
     }
     //Function for Spawning
     private void HudButtonsUsage()
     {
         //This Must be the first Button on the HUD
         if (Input.GetKeyUp ("t"))
         {
             Instantiate(BuildQPrefab1, spawn.position, spawn.rotation);
         }
         //This Must be the Second Button on the HUD
         if (Input.GetKeyUp ("y"))
         {
             Instantiate(BuildQPrefab2, spawn.position, spawn.rotation);
         }
     }
 
     //Function for SimpleMouseClick
     private void OnMouseDown()
     {
         selectionByClick = true;
         selected = true;
     }
     private void OnMouseUp()
     {
         if (selectionByClick)
             selected = true;
         
         selectionByClick = false;
     }
 }
 

This is the button setup I Currently have..

and this is kind of what it looks like..