my ubuntu laptop is running Microsoft Defender

How does that even work? Just wine it?

A desktop running a low usage wouldn’t consume much more than a NAS, as long as you drop the video card (which wouldn’t be running anyways).

Take only that extra and you probably have a few years usage before additional electricty costs overrun NAS cost. Where I live that’s around 5 years for an estimated extra 10W.

Clean the PC and change the fans. Fans make noise when old.