I'm interested in your build as a second/test server for myself. Primarily to learn Ubuntu and run Crashplan. unRAID was my first foray into the Linux OS world and I've been impressed with the performance.
I like the Helios, but will probably choose something different for improved cable management.
You may find the fans dB to be excessive for the bedroom. A fan controller helps some. A possibility is disconnecting the large 230mm fan. I was tempted to do that when I had 3 drives installed. Replacement of the stock fans is another option you mentioned.
btw, I like your choice in power supplies, although it may not run as efficiently as a smaller PSU, Seasonics are solid, modular is a plus for cable management, and I will bet that your build grows to 10-12 drives!
One valuable addition to your build - UPS. I found mine to be invaluable in protecting the server, and avoiding those long parity checks - especially when they occur <1 day after completing a scheduled check.
Welcome to the forum and enjoy!
Edit: Remember to check the serial number on your Samsung drives and update the fw if necessary.