Howdy!
I am a newb at all of this game stuff, but I have done voice over work on occasion for broadcast, and have also done extensive advertising jingles and music composition. One thing you need to be clear on is that if you "contract" the voiceover talent, you need to be contractually clear about the pay (or non-pay) and any residual income that might be due. Generally if you want it to sound pro, use a pro. Pro's aren't cheap, but they are willing to work on a tiered system sometimes if it is an interesting project for them. What I mean by that is, the voice becomes an actor in the game, and obviously the designers and all the other techies that came up with the concept and code deserve the big chunk, but throwing a bone to the peeps that make it sound pro shouldn't break the bank and it would actually be - uhhh - fair. Dare I say, you have seven people coming up with a game that could be worth nothing or a whole sh**load, so my advice is check the egos at the door, share the prize and shrink the greed glands. And make sure you have contracts - or it will all come back and bite you, but not until you aren't expecting it and have money coming in. Good luck, and hey, maybe i could help you out if you need me to.