Combining the_excerpt with the_content

The key is to use user-defined excerpts, rather than auto-generated excerpts. For example:

<div <?php post_class(); ?>>
<?php
// If post has defined excerpt, output it here
if ( has_excerpt() ) {
    ?>
    <div class="first-paragraph-excerpt">
    <?php the_excerpt(); ?>
    </div>
    <?php
}
// Now output the content
the_content();
?>
</div> <!-- .post -->

You’ll need to adapt to your needs, but this will output a div with the Excerpt, if it exists, and then output the content.