As for the USB cable, the same applies as for Ethernet in the sense that any standards-compliant, working cable is fine. It’s simpler because there is no choice of „Cat“. Look on Amazon, Anker is known to build good ones but Amazon Basics should do fine. I am partial to braided ones purely for haptic reasons 
Same with the streamer, it only has to reliably receive digital data and pass it on. They differ on features but all you need is Roon Ready. The quality of the wifi can also differ between models but with Ethernet it’s irrelevant. Finally, the quality and reliability of firmware updates can differ - but it’s hard to tell because one can find complaints about anything online. I think the Zen had a bit of a rough ride when it was new. I think it’s probably fine now but I know no details about the current state of things. There is a long thread here, but focus on the new posts and not the old ones:
It’s probably the cheapest plug-and-play option, at least as far as I know. There’s also the Bluesound Node in a similar price range. I used that for a few months to bridge a gap during system changes and it worked as well, but if you search the forum you will find the odd complaint about problems after updates as well. Hard to tell because like I said one can always find someone with complaints.