I'm happy to help.
SAS has its own connector. You would need a mainboard or PCI card with it, special cables and drives. For SATA you should be fine with the "consumer" power and data sockets. There is a SATA sockets witch combines 4 ordinary SATA interfaces. I'm not sure how its officially called, but you can get adapters for it.
Starting with 2U you should find cases that could fit a normal ATX PSU. The problem I see with the 2U cases is, the cooling. If you stack the cases on top of each other, the air intake would be blocked. The 3U cases are high enough to flip the PSU by 90° so the air intake is from inside the case.
For smaler PSU I found the Flex ATX size. They should fit inside even a 1U case.
You can get empty server cases, I guess even in the UK. Search for server case or 19" case. These come in different heights. Starting with a height of 2 or 3 U you can insert decent sized and therefor silent fans.
For the WAR depending on your DIY abilities you could build your own closet, add some 19" rails to an existing closet or get a small 19" rack
Take a look at the documentation of Lemmy.
The server-to-server communication uses the ActivityPub protocol
For backup you can look into rclone. It's what TrueNAS is using. You can set rclone up to only upload your data encrypted to a lot of storage providers like BackBlaze, Google, Dropbox, S3 etc. So you don't have to trust the provider with the ~~privacy~~ confidentiality of your data.
But anyone can set up an instance and become admin 😉