Unity has nicely designed built in functionality for triggers. You probably want a sphereCollider, marked as a trigger. See sphere collider docs

Then, assuming you are using a characterController or other rigidbody to move, when that object enters the sphere trigger, a message will be sent calling OnTriggerEnter on all scripts attached to the characterController, and the trigger.

As for controlling the opacity, you can set the color of GUI by calling GUI.color

You can use the Update() function in a script, which is called every frame, to compare the object's distance to the player with an amount you specify.

The easiest way is to just add the script to each object that could trigger the buttons (it is not the most efficient way, but if there are not that many objects running the script it won't matter).

Here's a rough idea. Add this to any of the 'approachable' objects, make sure you Tag your player object "Player", and give it a whirl.

using UnityEngine;
using System;
using System.Collections;

public class Approachable : MonoBehaviour 
{
    public float approachDistance = 5;
    public float buttonDrawOffsetHeight = 1.0f;

    [Serializable]
    public class MovieButton
    {
    	public string buttonText = "Play Movie";
    	public string movieName = "movie.mpg";
    }
    public MovieButton[] movieButtons = new MovieButton[1];

    private Transform playerTransform;
    private Transform myTransform;
    private bool drawButtons = false;

    void Start () 
    {
    	// find the player object - just do this once when we start up - assumes you've Tagged your player object "Player"
    	GameObject playerOb = GameObject.FindWithTag( "Player" );
    	if( playerOb != null )
    	{
    		playerTransform = playerOb.transform;
    	}
    	else
    	{
    		Debug.LogWarning("Could not find player object!  Turning off!");
    		this.enabled = false;
    	}

    	myTransform = transform;
    }

    // Update is called once per frame
    void Update () 
    {
    	if( !playerTransform )
    	{
    		this.enabled = false;
    		return;
    	}

    	// can compare squared magnitudes - a bit faster
    	float sqrDist = (playerTransform.position - myTransform.position).sqrMagnitude;
    	// square the distance we compare with

    	drawButtons = ( sqrDist < approachDistance*approachDistance );
    }

    void OnGUI()
    {
    	if( !drawButtons )
    		return;

    	// only draw if we're on screen
    	Vector3 buttonPos = myTransform.position + new Vector3(0, buttonDrawOffsetHeight, 0);  // adjust world Y position 
    	Vector3 screenPos = Camera.main.WorldToScreenPoint(buttonPos);

    	// render only if it's in the screen view port	
    	if (screenPos.x >= 0 && screenPos.x <= Screen.width && screenPos.y >= 0 && screenPos.y <= Screen.height && screenPos.z >= 0 ) 
    	{
    		Vector2 pos = GUIUtility.ScreenToGUIPoint(new Vector2(screenPos.x, Screen.height - screenPos.y));

    		GUILayout.BeginArea( new Rect(pos.x, pos.y, 100, 300) );
    		GUILayout.BeginVertical();
    		foreach( MovieButton mb in movieButtons )
    		{
    			if( GUILayout.Button( mb.buttonText ) )
    			{
    				Debug.Log("Play this movie: " + mb.movieName);
    			}
    		}
    		GUILayout.FlexibleSpace();
    		GUILayout.EndVertical();
    		GUILayout.EndArea();
    	}
    }

}

I can't test this solution, because I don't have Pro to use movie textures. Whatever you want to be displayed in teh window belongs in the WindowFunction. Thus, your button controls belong there as well (unless you want them to be drawn besides the window). I'd add the buttons to control the movie texture within the appropriate window function like this (add this excerpt into the code above):

function WindowFunction (windowID : int) {

   if (GUI.Button (Rect (10,20,100,20), "Play"))
      renderer.material.mainTexture.Play ();
   }
   if (GUI.Button (Rect (10,100,100,20), "Stop"))
      renderer.material.mainTexture.Stop ();
   }
}

It seems like triggering the buttons based on Brian's answer seems to work for you. So I modified my script to reflect the trigger functionality. I basically replaced the Update function with OnTriggerEnter and OnTriggerExit functions. Rest stays the same.

//attach this script to your first person controller
//make sure your target has a sphere collider attached which is set to "isTrigger"
//set the radius of the sphere collider to the distance which you want as your proximity-check

private var buttonOn : boolean;
private var windowOn : boolean;
private var windowRect : Rect = Rect (150, 150, 120, 50);//window parameters


function OnTriggerEnter (other : Collider) {
    	buttonOn = true;
    }

function OnTriggerExit (other : Collider) {
    	buttonOn = false;
    }


function OnGUI () {

        if (buttonOn == true) {

                if (GUI.Button (Rect (25, 25, 100, 30), "WindowOn")) {
                        windowOn = true;
                }

                if (GUI.Button (Rect (25, 100, 100, 30), "WindowOff")) {
                        windowOn = false;
                }

                if (windowOn == true) {
                        windowRect = GUI.Window (0, windowRect, WindowFunction, "My Window");
                }
        }       
        //take this "else-statement out if you want the activation of the window to be remembered, even if you leave the proximity of the object
        else {
                windowOn = false;
        }

    }

function WindowFunction (windowID : int) {
        // Draw any Controls inside the window here
    }

I'm rather new to scripting, so it might not be optimized or clean, but it should give you an idea of what you could do. It works for me. Attach the script to your first person controller and drag a target GameObject on it in your editor.

//attach this script to your first person controller

var targetObj : GameObject;//drag your target into this slot in the editor
var proximity : float = 3;//change trigger proximity here or in editor
private var buttonOn : boolean;
private var windowOn : boolean;
private var windowRect : Rect = Rect (150, 150, 120, 50);//window parameters


function Update() {

    	var dist = Vector3.Distance(targetObj.transform.position, transform.position);

    	//check whether you are within target proximity
    	if (dist < proximity) {
    		buttonOn = true;
    	}
    	else {
    		buttonOn = false;
    	}	
    }

function OnGUI () {

    	if (buttonOn ==true) {

    		if (GUI.Button (Rect (25, 25, 100, 30), "WindowOn")) {
    			windowOn = true;
    		}

    		if (GUI.Button (Rect (25, 100, 100, 30), "WindowOff")) {
    			windowOn = false;
    		}

    		if (windowOn == true) {
    			windowRect = GUI.Window (0, windowRect, WindowFunction, "My Window");
    		}
    	}	
    	//take this "else-statement out if you want the activation of the window to be remembered, even if you leave the proximity of the object
    	else {
    		windowOn = false;
    	}

    }

function WindowFunction (windowID : int) {
    	// Draw any Controls inside the window here
    }