Take this advice from someone who's been there and had plenty of opportunities to make mistakes (and even learn from them!). You absolutely do not need voice actors. You do not want voice actors.
Voice actors are an unnecessary distraction. Instead of focussing on getting the job done, you focus on finding the right voice actors... but then, you only have half the scripts available for them, because you're not done with the design. So, you get half your lines recorded, and then, a year later, you're finally ready to record the rest - only, the voice actors who need to come back to continue their roles have gone missing! Suddenly, you find yourself rewriting scenes, writing characters out of the story, writing new characters into the story, and so on, and so on. And when you decide to make some small changes in the first part of the game, the one you already have voiceovers for... you realise that you can't make the change, because you won't be able to record the new lines.
You will need voiceovers right before the release of the game. If you are planning on having video cutscenes with talking heads, or any similar pre-rendered stuff, you'll need voiceovers a little earlier - but such cutscenes should be left to the end of the project, for the reasons described above.