Without a doubt, setup your second PC as a network iTunes server. Simply connect it via Cat5 directly to an unused port on your wireless router.
Now for your Denon, I'd connect it via Powerline Network Adapters. They are so simple to use and plug and play. You can use 100mb models if you don't plan on doing video at some point, but the price difference is so small I'd invest in the 500mb variety. I use the Netgear AV5001 units. The nice thing about Powerline is either unit in a pair can be used as a receiver or transmitter so if for example you buy two pairs, you wind up with the one required transmitter and three receivers resulting in three available zones of networked Audio/video.
You've indicated you have iPads/phones?........if so, the Remote App is an invaluable tool for streaming. You won't have to turn on a TV to operate the Denon's AirPlay, but use the IPad as the interface. There's tons of other things an iTunes network can do, but I don't want to complicate things.....best to concentrate on te core and get that working solid first........but it can be done. I currently run 7 zones throughout my home and yard.