A network player is usually a Roon endpoint, not necessarily a Core. The cheapest were explored in this thread. I think the FriendlyArm $7.99 NanoPi Neo holds the record. It’s not plug n play but there is a version of DietPi available that enables menu driven Roon Bridge installation.
Network players need an onboard DAC to output analog sound. The DigiOne doesn’t include one (listening to one atm, very clean SPDIF up to 192 kHz). There are a number of DACs available as HATs to Pi devices (not I think the Nano). Checkout HiFi Berry, IQ Audio, Pi2Design and Allo for various offerings. The Allo Piano has a 2.1/2.2 version with subwoofer channel(s) out.
The NUC units are higher powered and are able to run a Roon Core. ROCK Is offered by Roon as a stand alone operating system that just runs Roon Server and turns the NUC into an appliance.