You could always hop over to https://frame.work and pick up a refurbished Intel mainboard plus an enclosure (or 3D print your own using their plans). You’d just need to get some RAM and a small SSD to run an OS like TrueNAS and you’d be good to go for probably under USD $600.
Take a look at Minifree Ltd. For less than USD $500, you can get a decent ThinkPad with Libreboot and your choice of Linux distro (KDE Debian is installed by default).