My 2014 Mac mini has two internal hard drives because that era supported Fusion drives. Mine wasn't specced with a Fusion, but for about £10 I picked up an adapter from eBay so I could populate the NVME slot. As a result I've got a 1tb 2.5" SSD that houses /home, and a 250gb NVME drive that the rest of the OS lives on. But they could be set up in any way that suits.
The only real caveat with that Mac is to ensure the one you get has 16gb RAM, because it ain't upgradable (unless you're dosdude1). Also, it's GPU isn't much cop. But mine is running Debian and a bunch of services on 8gb and doesn't cause me any issues.