Unable to get_the_content(); of a post in WordPress via AJAX

get_the_content() must be used inside the loop, or otherwise you’ll need to use setup_postdata(); providing the post object.

Try the following:

function ajax_action_stuff() {
    $post_id = $_POST['post_id'];

    //Query the post and set-up data
    $post = get_post($post_id);
    setup_postdata( $post );

    update_post_meta($post_id, 'post_key', 'meta_value');

    //Return response as an array. This will give us data.title and data.content browser-side.
    $response = array()
    $response['title'] = get_the_title();
    $response['content'] = get_the_content();

    echo json_encode($response);
    exit;
}

This has not been tested