For a server, that is a computer that has the main program, memory and holds music files, the intel NUC NUC7i7BNH with 4- 8 GB of memory and 64-128 GB of SSD would be hard to beat for under $700.
For a transport with coax out,
The Allo DigiOne is considered very, very good.
Using their Sparky SBC with their USB Bridge could be the answer for USB output
USBridge bundleIf you are looking for a transport with analog output, Allo offers the
Boss V1.2 along with their
Isolator which is a galvanic isolator.
In my and many other's minds, it is best to keep the server out of the listening room and run ethernet to a transport. The Allo Sparky and RPi work very well for this function with a few add ons.
I run ROON Server on a Windows machine in my basement and I use an Odroid C2 using DietPi software to send USB to my DAC. I do hope at one point to be able to change to a NUC running ROCK. The Odroid C2 does not have a lot of the problems associated with the RPi but there is very little software support.