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..

Code (CSharp):

1.  using UnityEngine;
2. using System.Collections;
3. using UnityEngine.UI;
4. public class Buildings : MonoBehaviour
5. {
6.      //Buttons for Hud
7.      public Rigidbody BuildQPrefab1, BuildQPrefab2;
8.      public Transform spawn;
9.      //drag Selection
10.      public bool selected = false;
11.      //private Vector3 moveToDest = Vector3.zero;
12.      public float floorOffSet = 1;
13.      //Click Select
14.      private bool selectionByClick = false;
15.    
16.    
17.      // Update is called once per frame
18.      private void Update ()
19.      {      
20.          HudButtonsUsage ();
21.          //Place Layer Unit Control Here.
22.          if (Input.GetMouseButton (0))
23.          {
24.              if (!selectionByClick)
25.              {
26.                  Vector3 camPos = Camera.main.WorldToScreenPoint (transform.position);
27.                  camPos.y = CamOP.InvertMouseY(camPos.y);
28.                  selected = CamOP.selection.Contains (camPos);
29.              }
30.              if (selected)
31.                  gameObject.GetComponentInChildren<Projector>().enabled = true;
32.              else
33.                  gameObject.GetComponentInChildren<Projector>().enabled = false;
34.          }
35.    
36.      }
37.      //Function for Spawning
38.      private void HudButtonsUsage()
39.      {
40.          //This Must be the first Button on the HUD
41.          if (Input.GetKeyUp ("t"))
42.          {
43.              Instantiate(BuildQPrefab1, spawn.position, spawn.rotation);
44.          }
45.          //This Must be the Second Button on the HUD
46.          if (Input.GetKeyUp ("y"))
47.          {
48.              Instantiate(BuildQPrefab2, spawn.position, spawn.rotation);
49.          }
50.      }
51.      //Function for SimpleMouseClick
52.      private void OnMouseDown()
53.      {
54.          selectionByClick = true;
55.          selected = true;
56.      }
57.      private void OnMouseUp()
58.      {
59.          if (selectionByClick)
60.              selected = true;
61.        
62.          selectionByClick = false;
63.      }
64. }

This is the button setup I Currently have..


and this is kind of what it looks like..

 

So Im Guessing noone knows? so its not possible? so only way is to draw buttons from script for each building click which is actually retarded.. whats the point of the GUI system them..