First, thank you all for your prompt and informative replies.
Someone asked what my budget is. I should have stated that up front. I'd like to keep this purchase at or under $800.
Someone else asked if there is a Microcenter nearby. There is not. But please keep in mind, I know just enough about adding/replacing/upgrading components to a PC to get me in trouble. I know there are resources I can use (YouTube videos, etc...) but my preference is to buy a machine "as is" that fits my needs.
Someone else asked if I have considered NAS, Synology or QNAP. I have not. Mostly because I don't know anything about that technology (I had to look up two of the three terms).
Someone else mentioned needing a USB video capture device if I planned on using Plex on my Fire Stick. If I use Plex with a 4K Roku (I think it's the latest Roku version on the market), will I need to tweak Plex as I would if I use it with a Fire Stick? If not, then Plex and Roku it is.
I have to go to work now. So it will be several hours before I can check back and respond to any new replies. Again, thanks to everyone for taking the time to share your suggestions.