I wouldn't necessarily recast the voice actors as the on-screen counterparts as a matter of course, but in the case of Hackett and Anderson I definitely would. Both Lance Henriksen and Keith David are veteran actors who could pull off their parts with aplomb.
I would also cast Brandon Keener and Steven Barr to voice Garrus and Wrex, since I assume they would probably be a mix of CGI and practical makeup. The on-screen performer could do the part, then have the voice actors come in and overdub them. The voices for those two characters are as much a part of bringing them to life as anything.
As for the story, ME1's was fairly tight and straight-forward. I think it would be possible to adapt it without having to change too much or cut any important characters and still get it into a 2-3 hour running time.
If they were to do it, I would hope they would treat the subject matter with respect and maturity and not play anything for laughs or winks.
And they would have to make choices to tell the story. There's no way around it.
Assuming they referenced the in-game choices at all in their version, I imagine they would let Wrex live because I'm guessing he would be a fan-favorite character. They would probably kill Kaidan because he's not an interesting character and from a business standpoint, Ashley is more marketable. Those are about the only real important choices that are directly tied to the plot if they were doing a close adaptation.
But it all might be moot. There hasn't been a word about it in a long time. Which might be for the best. The potential is there to really screw it up.