Re: Forums as Comment engine?
Sadly WordPress/bbPress integration isn’t well defined anywhere outside this website – it basically means shared users and login (though for alot of people thats ideal).
If you have the cash to spend on something that already works, I’d definately favour that option.
The idea of extending bbPress into WordPress is a good one, but of course you’d still need to edit WordPress to either post comments to the right forum, or to redirect to the right forum if someone tried to add a comment. I’d be surprised if there was much desire for this on the whole, so going with the existing solution might be your best bet.
Good luck in whatever you decide, and let us know