I have the Terra-Master 541 NAS. It cost about £540. I'm using 10tb drives and they are £300 each. I also upgraded the RAM, from 8gb to 16gb for about £50. It was a lot cheaper to buy it separately and do it myself than to buy it already upgraded. A NAS is a mini-computer, so that's why it costs more than a storage system that really only needs RAM and power.
Seedboxes, like any subscription (except gyms), should continue for the full term, even if you request a cancelation. Say you pay monthly, but you decide to cancel with 15 days left that you paid for, you should be able to keep using it for the remaining 15 days. I currently use Seedbox Solutions for my seedbox.
My husband works in Norway during the week, so he uses it so he doesn't have to pay for Norwegian tv. I have TV, movies, and porn on there. He also uses my Netflix and Amazon Prime accounts, as well.
We have 3 TVs and various other devices that we can view videos on, so a single storage device is so much easier. No carrying around external HDDs to watch a movie upstairs.