Another source of RAM is Crucial. You can input your model and they will suggest options that come with guarantees they will work. I you are not overclocking, (and you cannot overclock a NUC) then pretty much the cheapest option should be OK.
DDR4 2400 doesn’t run at 2400mHz speeds when it is installed, but only at the standard DDR4 speed of 2133mHz (thats is for all DDR4 speeds you might buy). You would need to be able to set “XMP” in the BIOS for the memory to run at 2400mHz. I’m not sure if you have that option in a “normal” NUC. The Skull Canyon ones do but they aren’t supported for ROCK (and frankly it would be unwise to choose one of these).
Also, modern NUC’s support NVMe and SATA III interfaces for their M.2 SSD’s. They look the same, but NVMe is considerably faster. That is -for as far as I can read- the only thing you you need to care about when selecting an OS M.2drive for your new Roon Core NUC. 64GB NVMe (also called PCI-Express disks) exist, but there are more options in larger capacities.