If audio is all ur interested indeed an HTPC is overkill and needless hassle for build and maintenance.
U asked why not just use PC to play instead of yet another box... because ur PC sits at a fixed location, do u have existing wirings that goes from this PC to the rest of the house? and do u ONLY intend to listen to your music at ONE location?
Lastly, nobody ask this, how EZ is the interface that will let you pick a song from your vast collection easily and quickly? What's the search capability looks like? Would you like to see album covers? Wouldn't you prefer to control it from a hand-held device rather than have to walk up to the box and push buttons (so 20th century)?
B4r u sell the 400-disc changer, rip all to FLAC (or whatever format of your preference) first, because then the process can be potentially automated, THEN unload the changer.
Solution: FREE. Explanation: I will have to charge$ you.