Just about any computer built in the 21th century is capable of being used as a simple file server
Indeed.
I bought a secondhand MSI 135 10" netbook at the weekend, for £17. It's got a 60GB SSD and 2GB of RAM, running Win7 Ultimate.
I installed MusicBee, and it will happily act as a DLNA server, or DLNA controller. In the latter case, it takes about 5% CPU casting to my DLNA renderers around the house, from my NAS file server. I also installed BubbleUPnP Server, which will create a proxy DLNA renderer to re-cast to Chromecast Audio devices and groups.
I also have an RPi running Volumio, accessing the NAS.
I also have a £35 Android media box connected to the TV, running BubbleUPnP, that can also at as a full DLNA suite (DMS, DMC, DMR).
The NAS also provides a DLNA server. As can any of the Windows PCs around the house. In fact, I explicitly had to disable the Windows Media Server service on the netbook, as I've got too many servers appearing on my network, and I only really need one: the NAS....
Oh, and my router has a USB port on it. If I connect a HDD to it, it will create a file server and a DLNA mdia server from that disk... That's probably the cheapest file/media server option you've got, and routers are usually 24/7, or, if on a time control, your servers will go down when your router goes down; and you wouldn't be able to access a 24/7 server without a router...