Sounds like a good plan. The N100 would almost certainly be more power efficient, bear in mind that a computer like this you will leave running 24/7.
Yeah, don’t back up games. You can easily redownload them from Steam should you lose them. No need to uninstall them, you can simply exclude those folders in your backup software.
Yep, that’s what that means, and yep, that should work fine. You do want redundancy, so either 2x12 in RAID1 or 3x8 in RAIDZ1 (which is basically the same as RAID5, so two drives storage and one parity) is what I would do.
I only recommended that size because it was the most storage per money last I checked. You can go bigger or smaller, whichever makes the most sense to you. I do recommend sticking specifically to Enterprise/NAS drives like the N300 though, they’re much higher quality than standard consumer drives.