Yes it is possible and absolutely viable as a starting point. You might will start experience some drawbacks of hosting on a Windows desktop system more sooner than later. Reboots due to system updates as example.

Use it, and think about getting an use office PC that can hold your drives.

Why would you do this? It would increase risk of system failure by reducing component redundancy.

If you just want to be able to access the files on your desktop when you're not using it, enable file sharing to the local network.

But the best solution would be to either buy a cheap hardware NAS, or build one from old components.

I think that should be possible but I would prefer the second option mentioned - Hyper-V role with a NAS OS Vm and drives passed through to it. Then collect in RAID inside a VM.

It's absolutely possible. My recomendations: Do not boot an OS from a USB stick, USB sticks are made with recycled donkeycum and will quickly fail when reading/writing as much as an OS neeeds to. I'm sure there is a boot loader that can help you choose wether to boot into windows or your NAS OS everytime you reboot.