Absolutely anything can be turned into a NAS, as long as you’re aware of your own needs and the hardware’s capabilities. A NAS is just a computer with some specific requirements.
When I first built my NAS, it only used parts that I got for free. A cheap micro ATX board with only two RAM slots, an i3-4160 CPU, 2x2G RAM, a worn-out SSD, and a 1T HDD. It couldn’t run something like TrueNAS, but it was enough for Proxmox and some Alpine containers running services like Samba, Transmission, Wireguard, and a small Debian VM for me to fuck around with. The single storage disk means there is no redundancy, so I only store replaceable data on it, like TV shows and installers.
There are many hardware-focused channels on video platforms that offer guides for budget home servers. Wolfgang’s Channel is good, and Hardware Haven and Raid Owl just finished a competition of building a sub-$200 home lab.