You can learn a LOT from HN itself.
HN is heavily moderated and the best part is a lot of people collectively do it and everyone accepts it. That's how the quality is maintained. You need to enforce this from the beginning. Quora did it and has quality. Yahoo! Answers didn't, and doesn't have that professionalism that Quora has.
Also, HN has quite a complex rating algo in place for upvotes. You might want to be inspired with that too.
Are you planning to give rating to users based on a voting strategy? It usually attracts people to be more interactive and get more visibility on a platform.
From my own experiences in managing a community/platform, concentrate more on the quality and quantity will increase gradually. Getting a lot of people joining initially itself can open roads for destruction in the future, which you might not be able to see now.