I think one of the problems with voice acting in games is the way it tends to be recorded. They hire expensive professional actors, who sit in a booth and read through hundreds of lines without much context or grounding as an actor. That often comes through in the results.
I would consider getting actors from the local college or amateur society, who would work much more cheaply. Then you could afford the time to get them to do workshops, play scenes off each other, work with the director - the stuff you'd do before putting on a play. I think you could get a much better result. There is no shortage of aspiring acting talent out there