HelpScout, ZenDesk or Desk.com.

HelpScout (help desk software) would be a great start – less options but even when I ran a 6 person customer support team (with over 500 emails a day), we never needed the complex features of Desk.com.

Have one person create 'saved replies' to quickly answer your customers in a cohesive voice and with consistent responses. (However, encourage your entire team to inject some of their personality!) This seems like a huge task, but if they dig through their sent emails, it can be done pretty quickly.

Good luck!