

While any machine that provides storage through the network can be a NAS, you’ll probably want at least some level of extendability. Your biggest problem with OEM machines (think Dell, HP, Lenovo) will be s lack of SATA ports, in combination with not enough power outlets and not enough space to put the actual disks in the case.
That’s the reason I usually build my own machines for those purposes, and depending on what’s needed you don’t even need high-end parts, at least for a “NAS-only NAS”. My NAS works on a 2 core/ 4 thread Intel i3, which will be enough for the foreseeable future. But it also only provides storage, all the rest (self hosting my services) happens on another machine.
TrueNAS works well for me as a NAS OS. AS far as I’ve seen it also provides direct container support or virtual machines. It you want an all-in-one machine, that might be worth looking into.
Which isn’t a great idea with all the breaking changes. I’d assume it gets better after v2, but still.