bbPress.org

If I understand correctly, you are trying to get the first post of the topic in its entirety. If that is the case, use the function bb_get_first_post to get the first post for the topic, which is what is used by the RSS feed. Then, use the post_text function to get the text for the post based on post_id. Using your example above, you would use:

$post = bb_get_first_post(get_topic_id());

$post_text = post_text($post->post_id);

Whenever you can, you should use the built in functions instead of calling the db directly, unless you have a valid reason for doing so. When you use the built in functions, the appropriate filters are called so that any installed plugins can work (One example would be adding rel=no-follow, or target=blank within your links). Only if you are trying to avoid running the plugins, or you are doing something completely custom, should you not use the built in functions.