Is this server just going to be a computer feeding a USB DAC? If so, your best bet would be to use a laptop, since a laptop doesn't require an additional monitor, keyboard and mouse. A laptop from Dell would work as well as most, but you could also find something inexpensive at Walmart, Costco or Best Buy.
On the other hand, if you go the Squeezebox route to computer audio then the sever can be just about anything, since it can be located anywhere in your house as long as its on your network. It can easily be run 'headless' (no monitor) by using remote desktop software such as VNC. For this type of server, a laptop is usually more expensive than need be. You can use a very small Intel Atom or AMD Zacate based computer that sits off in a corner or in the basement.