Still needs some more information.. What is the Server supposed to do? Provides user authentication? Serve files? Serve printers? Share an internet connection?
First, a Windows domain doesn't have much anything to do with controlling network traffic or internet usage. It gives you centralised control over user accounts and rights, plus, in case of true Active Directory, a place you can use to look up data like peoples phone numbers, ability to automatically install software and control some Windows settings in every computer/for every user..
If you set up a Linux machine, you get Samba, which at the moment gives you centralised user management, file and printer sharing, login scripts. This would be the "domain" bit.
if you use the same server to share the internet connection, then Linux lets you control the network traffic to infinity. But since you are asking this question here, you probably don't have the know-how..
As for the backup, any kind of a special sever would be a total overkill, unless we are talking about terabytes worth of data you want to archive.
For disaster recovery you could simply use two (sets of) external hard drives. One day you use set A, the other day set B.
If you need to archive data for years, use a DVD burner, store the hard drives themselves from the enclosures or invest in a simple tape drive.
Maybe elaborate a bit more? Tell us exactly what you have and what you want to do?
