I don't think that's relevant to the thread and the OP is not required to answer that question. More over, the OP not answering that question doesn't mean that he can't be given advice on setting up his seedbox. Everything is within the rules so long as we only talk about the machine running torrents, not from where the torrents come from or their content.
For the board, id get something like a intel celeron and a board like this one https://www.amazon.com/GIGABYTE-LGA1151-Intel-Motherboard-GA-H110M-/dp/B0165YUDTM?psc=1&SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-ffab-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B0165YUDTM
If you need more sata ports, add a card like a dell h200 later on.
For cpu, id get the cheapest celeron for that socket, its more than fastenough here and is very low power. look at a g3900
Get 4gb of ram.
Case is where i gets interesting, id just get a basic desktop case, normally you can put 4 3.5 drives in it and then you have some 5.25 bays for some 3.5 adapters. You can also just get a nas case like the unas, or something like a node 3.04.
For os install drive, id just use a usb stick, or 2 in raid 1 if your conserned about reliabity.
For os, if you comfortable with linux, id just run centos(insert favorite linux distro here) and configure it to your liking. If you want a easy to use nas os, id run openmediavault.