I believe forum digests only go out for new threads that are posted in a forum. So, if there are new posts to a thread in a forum, these will not be included in the digest.
As for what to add... I've never done what you want to do, so I can't really give you specifics. But, the query I pointed out is, I think, the one you need to modify to grab the first post text at the same time as it grabs the other info. You need to add a join, something like:
PHP Code:
INNER JOIN " . TABLE_PREFIX . "post AS post ON(thread.firstpostid=post.postid)
And then in the SELCT part of the statement, you need to add in "post.pagetext". That should grab the pagetest of the first post in the thread for you. Then you need to do something with it. In the code above the query you are modifying, the post text is selected, and then they do this to it to get it ready for the email output:
PHP Code:
$post['pagetext'] = $plaintext_parser->parse($post['pagetext'], $thread['forumid']);
So, you may need to add that to your code before you then try to insert it into your phrase (with different variable names, of course).... you will have to modify that phrase also -
digestthreadbit ?
I would suggest you play with this on your test site.