"I can't stress quality enough here, it really is paramount that these characters are believable...  "

Hi not to be a "wet blanket"
here but from your description it sounds like you would need the services of $$$Weta Digital®.. or $$Industrial Light& Magic®$$$

Seriously.

Poser/Daz is great for of Alot of things, but it sounds like this project might be a wee bit too ambitious  to even attempt with poser/Daz figures.

If you are the person I responded to in the other thread I already know you are not going to try and render this project in poser.

But frankly rendering the finals is the least of your potential problems right now.

"promo renders that look freakily accurate and will be working hard to emulate the facial expressions and bodily movements of the real people in the animation."

well poser/Daz cant really do this without
custom facial rigging with simulated under skin muscle controls
so if indeed you are planning to manually animate the characters in poser then you are in for much pain& suffering.

if you plan to use canned Mocap from outside sources you are in for a different kind of frustration.

If you plan to have these "realistic" & "convincing" figures created for poser and then send them to some other high end program via one of the many  a plugin/hosting options( FBX ,Collada Blah blah),

Be prepared to lose some portion of the poser functionality you paid to build into the figure in the first place( such as any poser native dynamics or skin shaders).
 
Or somehow add more controls essentialy re-rigging the characters in whatever high end app you plan to use.

As an animator myself I advise you this:

If you have not already done so ,establish a viable end to end workflow with, proof of concept, animation& lighting renders,using your existing M4/V4 figures
these renders should include lipsynch as well as body motion& hair motion test's
within the environments you plan on using.

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