Remember ... if you are a software shop then YOUR CODE IS YOUR MOST IMPORTANT ASSET. Don't screw around or experiment, and don't go cheap.
1- Buy a low end IBM/Lenovo server. Our esteemed host described in all kinds of detail his travails in building his own .. I would buy one pre-build.
a. Make sure it has a RAID controller and can support RAID5.
b. Get plenty of disk .. multiply your current need by 10.
2- Since VSS, you will need Windows. I recommend you get Server, even if it isn't required for VSS.
3- You need to deal with backup. There are a zillion options. I would start by getting a tape drive and backup software running. Once that is running you might want to implement a remote or disk-to-disk solution in addition. More info here and here.
4- You will want anti-virus software. Use whatever you use on the desktops.