Primary Domain Controller?

I’ve purchased a monster file server for the company and am planning on using it for working files off the network. I also need to set up a primary domain controller for the company with user names and passwords. Should I have this one server be it or should I look into an older computer to act as the primary controller? I’m planning on doing some auditing on some 60 workers. Also wondering if this takes up a lot of network overhead from information being sent from other file servers?.