He could get a really nice monitor, a top end computer, something to stream video to from the computer, a huge NAS and a sound system. I'm trying to be cheap as possible and put together a home network that allows sharing and backing up of all files (from MAC and Win), time machine backups, streams media to my TV, and have a quick computer for normal task and encoding videos. I know my task would be much easier with $5000 instead of using old parts and buying little as possible.
So my vote would be for a high end computer, but not ridiculous not needed things, and all the networking to surround it, making everything easier for the whole house. I would much rather access my media from my couch using my laptop, with my tv and surround sound. The computer is for working or playing and won't benefit much stand alone in an office. Maybe pay a professional to come in and set everything up including a server, home entertainment, and a top end PC all with tech support and warranty.