Walmart currently has a decent 6TB drive on sale for $160: https://www.walmart.com/ip/48735757
Put 3 or 4 of these in a Synology enclosure or attach directly in a uATX case. I use btrfs "raid1" which protects against any one drive failing and has filesystem-level checksums and compression, but people also say good things about ZFS in a similar mode. Just use either one of those with scrubbing.