Either a desktop/tower-type computer that’s as fast as possible with plenty of RAM or a NUC11 or newer with i7, at least 16 and maybe 32 GB RAM and running ROCK.
Generally, single core speed is the most important thing about the CPU. (Many cores is not so important unless you output to several zones). Don’t use server-type CPUs like Xeon that have lots of less fast cores.
For the desktop-type computer, use the OS what you are comfortable with. Windows, Linux, and Mac (e.g. a Mac Mini M3 if you can) are all fine.
The NUC models that are supported by ROCK are listed here:
Note that NUC13 is not yet on the list, but there are reports that they work very well with ROCK (but not the HDMI output).
If considering ROCK, you might want to wait a bit because Roon has said that they have a successor to the Intel NUC platform in the pipeline as the supported computer for ROCK. And they have just announced the Nucleus Titan as a very fast new Roon device. (Of course, if you have money to spare, you might consider the Titan itself, but it starts at $3.5K and you get a lot of computer hardware for this money if you don’t need it from Roon directly)
There are many forum topics about all of this.
Note that there are some limitations in Roon at least currently (which may or may not be improved) with certain library structures. With a fast computer you should be fine with 200K tracks anyway, but try to avoid this if you can:
Very large numbers of local albums that are “unidentified” by Roon because there is no metadata. (A typical culprit are, e.g., downloaded band live concert archives or bootlegs)
Single artists with 100+ albums or 1000+ tracks.
Many Albums with 100+ tracks.
Roon tags that are applied to hundreds or even thousands of items.
Examples for support cases and ongoing discussion about these topics are, e.g., here: