I want to build a dedicated computer solely for playing music digitally.
So Ben, let's try to start from here: why a dedicated computer?
If the only function will be audio playback, why not a dedicated player, something like the Aurender
As for aurender and similar devices, if a network player of that sort is implemented well, they might be equal or better than a computer outputting audio via usb. The only one I've liked so far is the naim network player. Rest haven't been impressive at least to me. Aurender - I can't comment as I've never heard one.
I beg your pardon, Ben, but I would appreciate very much if you may quote a tad more properly when answering.
Said that, my point is that a computer do require some manteinance and it is not well suited into headless operation, right out of the box: a player would be more practical for just playing files (moreover objects like the Aurender are file players or music servers, not the same kind of network players, they are sort of a small dedicated computers, like some dedicated rigs for coin mining).
If you want to stick with a PC, I would give a look to some dedicated enclosures, like the Origen or the Soundgraph, along with the already quoted HDPlex and Streacom. The Grandia and Crown series from Silverstone are solid choices, but I would consider the mATX Fortress too.
If you don't mind about software manteinance, I would give a look to operating systems with headless operations in mind: I am thinking to some projects like either Daphile or Vortex.
With reference to storage capacity, I have some doubt about the planned 2Tb as somehow conservative (the quoted Aurender player sports a total capacity of 8Tb). However, whether 2Tb were enough for your digital archive, given that cost is not an actual object and price aren't that high, I would also think to get rid of any mechanical disk, and I will use a bunch of around 1Tb Crucial 2.5" SSD, so that, for a no-moving-parts possible setup, I would think to something like a Silverstone Fortress FT-03, coupled with a NoFan CR95C cooler, and a Seasonic 400FL PSU (about active hardware, about any H97 or H87 mATX board from ASRock, ASUS, or MSI along with a Core i3 will be far more than fine).