I forgot about the "advice" part...
I would focus first on the data needs of this external community. Do they need specific access to *everything*?
Do they need access to just one site collection? If yes, that may be overall easier to manage.
Will you allow them to self-register?
Can they change their password?
How will you manage their access overall? One person to manage it or will individual site owners manage it?
I would story board it out and place yourself firmly in the shoes of these external users. Figure out what they would want/need to do in terms of security and access and only then dive into the technical bits.