Well if anyone else is trying to do the same, I have managed to at least remove the default forum search in order to force users to use the classifieds search. I edited the index.template.php and removed the following code at Line 351:
<form action="', $scripturl, '?action=search2" method="post" accept-charset="', $context['character_set'], '" style="margin: 0;">
<a href="', $scripturl, '?action=search;advanced"><img src="'.$settings['images_url'].'/filter.gif" align="middle" style="margin: 0 1ex;" alt="" /></a>
<input type="text" name="search" value="" style="width: 190px;" />
<input type="submit" name="submit" value="', $txt[182], '" style="width: 11ex;" />
<input type="hidden" name="advanced" value="0" />';
// Search within current topic?
if (!empty($context['current_topic']))
echo '
<input type="hidden" name="topic" value="', $context['current_topic'], '" />';
// If we're on a certain board, limit it to this board ;).
elseif (!empty($context['current_board']))
echo '
<input type="hidden" name="brd[', $context['current_board'], ']" value="', $context['current_board'], '" />';
echo '
</form>