@jmarshak -- you didn’t mention how many endpoints you plan to run simultaneously, and whether you are going to use DSP (e.g. Roon’s Convolution engine or parametric EQ). If your DAC is going to upsample to high-res formats like DSD, then that matters too.
If you are just starting out, or only have one streamer, and don’t plan to use DSP or DSD conversions, then IMO you can simply use a general purpose desktop or laptop as the Roon core.
But if any of the above applies, then a dedicated Roon core makes sense. You can build your own using Intel NUC, or you can buy from a vendor. In my case, I contemplated building my own NUC in a fanless configuration. But eventually went with Small Green Computer’s sonicTransporter i5, since it came out to be almost the same cost. It is fanless, acts as a heat sink and has been rock solid so far. You can order it with an SSD hard drive pre-installed or add one yourself very easily, no tools required.
Finally, as for getting a one-box solution vs. multiple boxes, I prefer the ultimate flexibility of a multi-box solution simply due to the fact that this space is still evolving quite rapidly. It gives me the flexibility to make incremental changes, for example try a new streamer without changing the core, or swap my DAC without changing the streamer. Of course, this is purely a personal preference and might not appeal to everyone.