I agree with Mark, a laptop is almost 100% of the time the wrong answer, unless you already own one or get one for free.
In either case, for a Plex server, the most important part is the CPU, and no, I'm not talking about 128 cores running 100 GHz, but what you need is the cheapest, but newest INTEL (Sorry, no AMDs as the video processing support isn't great) CPU that has built-in video processing. This is the key for Plex to easily and very efficiently transcode movies. One box with Intel N150 will be fine.
I highly recommend some Linux to run Plex on it; I use Ubuntu Server, since this is what I'm most comfortable with. 4 GB RAM should be plenty.
As for a client, I can't recommend Roku as they went the enshitified route. Nvidia Shield TV Pro is still good, but it's 7 years old design. I guess that leaves you with Apple TV 4K as your best bet.