I previously had a 4-bay Synology and loved the UI but couldn't deal with the price jump to an 8-bay. I discovered xpenology.org which allows you to bootload Synology software on x86 hardware.

I turned an old desktop PC into an 8-bay storage machine with a PCIe SATA card and now have all the flexibility I want, without the price penalty for Synology hardware.

The best place to find hard drives is https://diskprices.com/ which scrapes Amazon and a few other sites to show TB per £/$. It's almost always cheaper to buy a retail packaged external disk and extract the hard drive.

Note: Some hard drives once shucked from external cases won't turn on when attached to regular SATA power adapters due to mismatches in power connector specification. Easily solved by using a SATA power to Molex adapter, and connecting that to a Molex to SATA power adapter (SATA->Molex->SATA) as Molex does not support the 3.3V pin which is the issue.