I want to build a proper server with room for 40+ HDDs to move my media server to and have RAID 1. I know a lot about PCs and software, but when it comes to server hardware I have no clue what I'm doing. How would I go about building a server that has access to 40+ RAID 1'd HDDs?
If you really want a setup with that many disks, you might look into Ceph. It's intended for handling stupidly huge amounts of data spread across multiple servers with self-healing and other nice features. (As the name suggests it's a bit of a tentacle monster though.) One of my colleagues set up a deployment at work. It took a while for him to figure out how to get it running well but it's been pretty useful.