DIY Computer Audio Server or Melco N1Z/2EX-H60

I will be succinct and to the point .... get the Melco.
The odds of your being able to better the Melco going the DIY route are very low in my opinion.  And there is much more to it than just getting the right parts and wiring them up, but very much in how those parts are wired up.

I haven’t heard the Melco. My general opinion is that there is a performance advantage in separating the ethernet-to-USB rendering function from the CPU and mass storage. Optimizing the ethernet-to-USB renderering function frees you to employ a more or less standard, economical computer-grade CPU and storage(albeit enhanced at the ethernet interface with the LPS-powerd JCAT NIC.) In my case, I was led to an i5 NAS to support the Roon core and to provide scalable storage to 20TB to support lots of DSD files of vinyl recordings and ripped SACDs. The files are virtually indistinguishable from the sources.
HOWEVER, all decisions are made at a point in time.  The latest generation of standalone purpose-built servers like Melco, Innuos, etc. are so good that it is tempting to go with simplicity.

Thanks again guys.Really appropriate the feedback.