Hi John,
Whilst the Celeron® 2.41GHz dual-core processor in the TS-251 will run Roon, you have a very large lib and you also want to upsample to DSD… I believe you should be looking at an Intel i5 based NAS.
Have you read What is the ideal NAS configuration for Roon?, it was written about a year ago so the models may have changed slightly.
One cool feature of Roon, if you open up the signal path window Roon displays how well the processor is coping vs. real time … as you have the TS-251 try upsampling and see what you get.
X5 = Means the DSP is running 5 times faster than it is required. As the number approaches X1.? problems will ensue.