Will home directories be mounted off the file server, or will the file server usage be somewhat more limited?
Samba can run as a Primary Domain Controller, but not as an Active Directory Server yet (coming in Samba 4).
Your hardware and other choices would depend on usage patterns, uptime requirements, disk usage per user, etc. For Linux, I would look, at a minimum, at a 2.5 GHz+ Dual or Quad-core Xeon, 8GB RAM, RAID 6 or similar for the hard drives, etc. Something along an IBM x3650.