Man, almost anything now. Transcoding isn’t really needed very often since streaming sticks (Roku, AppleTV, Fire, etc) now reliably have hardware video decoders. So, almost all the NAS has to do is shove the file over the network. Almost the only way you’d need transcoding is if you wanted to stream video out of your home network to remote devices.
So, if my guess that you won’t need transcoding is correct, then just buy the cheapest NAS that has a couple drive bays.
(There is such a thing as too slow. I have a 2014-vintage Synology DS214se, which was the low end model even then, and Plex clients take up to a minute to load all movie posters, and it can take a minute for a video stream to start. But if you are buying something made in the past few years this won’t really be an issue.)
The Synology DS220j would probably be my pick for that. Or maybe the 120j if you don’t care about data redundancy.
This week I discovered that Porkbun DNS has a nice little API that makes it easy to update your DNS programmatically. I set up Quentin’s DDNS Updater https://github.com/qdm12/ddns-updater
Setup is a little fiddly, as you have to write some JSON by hand, but once you’ve done that, it’s done and done. (Potential upside: You could use another tool to manage or integrate by just emitting a JSON file.) This effectively gets me dynamic DNS updates.