if it really is a data server, then I'd guess you'd concentrate the cash on hard drives, big ones, with raid, maybe a raid 1 (mirror) of a raid 0 (stripe), which you can do with 4 hard drives. Then a fifth one as a system disk. You don't need to back this up as all you will have is the bare OS because this machine has the function of serving disk space, nothing more.
Get a really cheap motherboard, but get it a raid controller card and lots of ram, spend the rest on hard drives.
If you want a data server, that is.
Spend no money on graphics cards, cool sound, etc. No fancy cases. But a big, ugly grey box that you are going to put 5 hard drives in. Try to get a big case that you can fit more drives in in future.
You would connect to such a machine over a network connection most of the time, its job to provide disk space and that's about it.
On your other machines, you map a share of the raid 10 volume as a drive letter (or drive letters) on your other machines.