I personally think that 90% of the actors mentioned would make horrible SM. While their voices would be good for a CGI movie like Beowulf, they would make crappy Primarchs. They are all skinny and girlie lookin', a SM, a PRIMARCH is a huge, battle scarred, bad ass mothafucka. I just can't see most of hollywood being able to pull it off, I shouldn't be able to beat up the Emperor!!!! Give some of the guys in the WWE and the UFC some acting lessons and we will talk.
Well, CGI would probably work. It looks like it's gonna work for Ultramarines, why not Horus Heresy?
Although voices could be a problem. I'm not sure if this is just me, but some sort of British accent sounds more noble than an American one. (Probably because most of the heroes in high fantasy epics have British accents) A sort of cross between them could work as Space Marine-ish enough, or a very gruff voice for the buffer, more savage Primarchs (coughrusscough) would be great.
And in answer to the problem of the sheer, massive amounts of info in the HH series, a series of movies would be the greatest solution. Either have a really great, big, 15-movie long series, or pick 6 or 7 of the best and see if it's successful enough to merit a few more movies. (That means at least 8 movies, because the end is going to HAVE to be told when it comes out)
That leaves us with the question of which stories, alongside the original question posed by the thread.
All I know is, the Fall of the Angels is going to have to go in there.
BTW, if a HH movie does get made, then what's the bet that people start calling Lion "Lionel Jonson"?