Are you Over heating with windows issues and have to pay loads to run your business ??. Well Open source is your solution,Run Zentyal to replace your existing Active Directory ,DHCP & DNS server, zentyal is a Linux base Operating System that can do much more than A windows server can. You can host your email on zentyal ( zentyal as a mail server).
still unsure. try this one Pfsense as Firewall, Pf is one of the most advance firewall, it is available freely on internet but require a good knowledge of UNIX to configure it.( which i think discourage people to use it as it is not everyone`s cup of coffee to configure PF). Still Unsure. try samba on Ubuntu server or Debian for your file sharing with ACL..
Visualization is the core method to over come huge budget that consume IT department. we propose you ESXI from VMWARE that is free and do much pretty job than Windows Hyper Visor. If you are looking for a free and scable way to virtualise your system then ESXI is right for you.