Just my read on this ...
I would not use something like the forum as a base, there is just way too much you don't need in there. And even though it might get you up and running faster, it will be harder to maintain and extend freely.
I think using one of the open source mailers you recommended would be a better option. I can see the attraction of having an in-Moodle mailing list manager.
I would add a third Strategy which is to use a mailing list provider, and build a neat Moodle API integration. The provider would probably be very happy to have you do that too.
Personally I use MailChimp. On one Moodle site I started, and never finished, I added a simple integration when users sign up to Moodle, such that they also get signed up to the MailChimp mailing list. You would also need a sync'ing tool. But they have a great API designed for this sort of job. And the benefit is they handle almost everything, and have a range of tools, iphone apps available
I am sure you have your own requirements though. That is just what works for me.