Wondering if I could get some advice for a simple home setup?

Reading through posts here I sometimes feel like I do not belong, but my setup is honestly pretty simple. I run Plex off a mini OptiPlex in my living room and all my media lives on an external hard drive. Lately I am running out of space and I need to expand, which brings me to a question for the group.

From what I have read, NAS drives seem ideal for a home Plex server.

With a budget of about $250–$300 I can either buy an 8TB NAS drive plus a basic enclosure, or I can get a single 20TB external from the usual brands. For example, 20TB desktop externals from Seagate/WD sometimes show up around the $230–$280 range, while a quality NAS-rated 8TB drive (IronWolf, etc.) is commonly priced in the same ballpark and you would still need an enclosure or NAS box. 

So here is what I am trying to decide: do I bite the bullet and grab the larger single-drive external for the raw capacity and lower cost per TB, or do I invest in a smaller NAS-focused setup. For Plex use on a one-person household, is the extra cost and complexity of NAS worth it, or should I just get the 20TB external and be done with it?

Thanks in advance for any real-world experiences, gotchas, or recommendations. If it helps, my Plex usage is mostly streaming 1080p rips and a growing collection of movies. No large transcoding farms or remote heavy simultaneous streaming.