External Search using an RSS feed

Disclaimer: I'm still very new to this, so this may be a stupid question. Most of my experience is in Java programming.

I need to build a module that will let me search my website with an an older external search engine which generates an RSS feed with the results. It uses URL-based queries, so it shouldn't be a big issue to implement, but this is giving me a lot more issues than it should.

Ideally, I would be able to replace the built-in drupal search to use the external search and display the results the same way they show up with the built-in search. I have a multitude of advanced search options I'll need to set up as well for full support.

Here's the module so far. I stole a lot of the code from the core Search module.

<?php
/**
 * Implements hook_help.
 *
 * Displays help and module information.
 *
 * @param path 
 *   Which path of the site we're using to display help
 * @param arg 
 *   Array that holds the current path as returned from arg() function
 */
function mndmf_search_help($path, $arg) {
  switch ($path) {
    case "admin/help#mndmf_search":
      return '<p>'.  t("Keeps the user on the MNDMF site after entering the search query.") .'</p>';
      break;
  }
} 

/**
 * Implements hook_permission().
 */
function mndmf_search_permission() {
  return array(
    'administer search' => array(
      'title' => t('Administer search'),
    ),
    'search content' => array(
      'title' => t('Use search'),
    ),
    'use advanced search' => array(
      'title' => t('Use advanced search'),
    ),
  );
}


/**
 * Implements hook_block_info().
 */
function mndmf_search_block_info() {
  $blocks['mndmf_search_en'] = array(
    'info' => t('MNDMF Search (en)'), //The name that will appear in the block list.
    'cache' => DRUPAL_NO_CACHE, //Default
  );
  $blocks['mndmf_search_fr'] = array(
    'info' => t('MNDMF Search (fr)'), //The name that will appear in the block list.
    'cache' => DRUPAL_NO_CACHE, //Default
  );
  return $blocks;
}

/**
 * Implements hook_block_view().
 * 
 * Prepares the contents of the block.
 */
function mndmf_search_block_view($delta = '') {
  switch($delta){
    case 'mndmf_search_en':
      if(user_access('access content')){
        $block['content'] = drupal_get_form('mndmf_search_box');
      }
	  break;
	case 'mndmf_search_fr':
      if(user_access('access content')){
        $block['content'] = drupal_get_form('mndmf_search_box');
      }
	  break;
  }
  return $block;
}


function mndmf_search_form($form, &$form_state, $action = '', $keys = '', $prompt = NULL) {

  $action = 'search/mndmf';
  
  if (!isset($prompt)) {
    $prompt = t('Enter your keywords');
  }

  $form['#action'] = url($action);
  // Record the $action for later use in redirecting.
  $form_state['action'] = $action;
  $form['#attributes']['class'][] = 'search-form';
  $form['module'] = array('#type' => 'value', '#value' => $module);
  $form['basic'] = array('#type' => 'container', '#attributes' => array('class' => array('container-inline')));
  $form['basic']['keys'] = array(
    '#type' => 'textfield',
    '#title' => $prompt,
    '#default_value' => $keys,
    '#size' => $prompt ? 40 : 20,
    '#maxlength' => 255,
  );
  // processed_keys is used to coordinate keyword passing between other forms
  // that hook into the basic search form.
  $form['basic']['processed_keys'] = array('#type' => 'value', '#value' => '');
  $form['basic']['submit'] = array('#type' => 'submit', '#value' => t('Search'));

  return $form;
}


function mndmf_search_box($form, &$form_state) {
  $form[] = array(
    '#type' => 'textfield',
    '#title' => t('txt'),
    '#title_display' => 'invisible',
    '#size' => 15,
    '#default_value' => '',
    '#attributes' => array('title' => t('Enter the terms you wish to search for.')),
  );
  $form['actions'] = array('#type' => 'actions');
  $form['actions']['submit'] = array('#type' => 'submit', '#value' => t('Search'));
  $form['#submit'][] = 'mndmf_search_box_form_submit';

  return $form;
}


/**
 * Process a block search form submission.
 */
function mndmf_search_box_form_submit($form, &$form_state) {

  $form_state['rebuild'] = TRUE;
  
  if (isset($_GET['destination'])) {
    unset($_GET['destination']);
  }

  if ($form_state['values']['mndmf_search_block_form'] == '') {
    form_set_error('keys', t('Please enter some keywords.'));
  }

  $form_id = $form['form_id']['#value'];
  $form_state['redirect'] = 'search/' . trim($form_state['values'][$form_id]);
}


function mndmf_search_theme() {
  return array(
    'mndmf_search_block_form' => array(
      'render element' => 'form',
      'template' => 'search-block-form',
    ),
	'mndmf_search_result' => array(
      'variables' => array('result' => NULL, 'module' => NULL),
      'file' => 'search.pages.inc',
      'template' => 'search-result',
    ),
    'mndmf_search_results' => array(
      'variables' => array('results' => NULL, 'module' => NULL),
      'file' => 'search.pages.inc',
      'template' => 'search-results',
    ),
  );
}

The block shows up fine, but giving a search query gives me these errors:

Notice: Undefined index: mndmf_search_block_form in mndmf_search_box_form_submit() (line 133 of mndmf_search.module).
Please enter some keywords.
Notice: Undefined index: mndmf_search_box in mndmf_search_box_form_submit() (line 138 of mndmf_search.module).

I'm not completely sure how drupal works with some functions and parameters, as I'm still pretty new to this. I can't seem to understand how $form_state really works as well.

Also if someone could point me in the right direction on how to request external XML data, that would be great. I have working source in ASP, but I don't have the slightest clue how to get that to work in PHP.

Thanks in advance :)