It absolutely is viable. I do so myself. There's pros and cons to this.
The cost of hardware is more because you have more pieces, you probably need a decent shelf to hold everything (check the local thrift stores for old home theater stands, they work great), and it may add latency for some things where they interact together.
The hardware modularity of it is really nice though. It's kinda nice not having to worry about losing all your data if you need to reinstall everything on the server. And it gives you the ability to update in smaller pieces rather than all at once.
That being said, a NAS itself generally isn't all that good at transcoding Plex. Most entry level NAS's ($200-$300 range) can only transcode a few streams at a time. However you might not use transcoding all that much anyway if you store media on it in a playable format.
I've personally only ever used Synology, and they're not bad, but they've been doing a few anticonsumer practices in more recent years that makes me want to get away from them. They're really easy to set up though, so it's not a bad option starting out.
If you're going to go this route, I'd host Nextcloud on the NAS as well as your Plex media, then mount the share on your server so that your server can do transcoding while you're NAS does bulk storage. Everything else could be hosted on the server
Considering you're starting from square one though, it may be better to just buy a beefier NAS, and then get a server as your needs expand. Especially if you don't plan on sharing Plex or Nextcloud with a bunch of people outside your home at this time.