I run a NAS as my PMS machine, but mine isn’t a low powered one, either. It’s only got a 5K or so passmark CPU in it, but it also has over 30TB of drives on it. (most of that external USB enclosures.)
As has been suggested above, without knowing what you want to stream to, what those client apps requirements are, and what media you want to stream, suggesting a server platform is, well, rather difficult. @cayars has a truly HUGE amount of media he streams from a PMS machine with about the same CPU specs as my NAS. He gets by with this because he optimizes his media to fit most of the clients that stream from him.
Ultimately, this is the way to successfully stream to many different clients across local and remote connections. Put the media into a format that is the most Direct Playable, and optimized for the bitrates your users are most likely to use. Then the actual loads on the server are minimal, instead of always pegging it out.
NASes can be a good intro to PMS, as they are generally fairly easy to set up. You don’t need to know a lot about the underlying OS to get PMS working. Most of the PMS options can be found in the Web App. But they can also have a steep learning curve to maximize the media you stream. Here is where you are going to have to struggle a bit to learn a few new things. Still, there are a lot of folks out there that have been down that road, and are willing to help people out.
My rule of thumb in this is simple. H264 only, MP4 only and at least AAC stereo English as the first audio track. Anything else is automatically converted to this before Plex ever gets it. This gets done through automation and scripts take care of a lot of the tasks. (Again, thanks to @cayars for this! his scripts are completely idiot proof! Hey, I got them working!) 
BUT don’t expect a $200 NAS to do what the heavier duty machines are capable of doing. Expect to spend at least $800-$100 for the BASE NAS before drives to get something that’s going to handle most streaming/transcoding anyone is likely to ask your system to do. (And, even though you may have maximized your media, you are always going to have SOME transcoding happening. It’s just a fact of streaming media…)
I suggest looking at Asustor (7004T is what I have), QNAP or Synology as these manufacturers build top quality machines, with CPUs that can do the job. I wouldn’t recommend anything from drive manufacturers, as those usually have a CPU that’s not much better than a repurposed cell phone CPU… (That means stay away from WD, Seagate, NetGear and similar NASes… They just don’t have the guts to do the job. My opinion, for what that’s worth…)
I consider my NAS pretty well future proof. The only thing I might need to do to it in the future is upgrade the socketed CPU with something a bit heavier duty. And throwing more drives onto it’s USB/eSATA ports as storage requirements increase. Other than that, it’s going to be just as capable in 3 years as it is now, for my needs. (No 4K content planned and I seriously doubt it’s going to happen before I need to think about replacing the NAS.) Of course I have close to $3K into mine, including drives, so…