As a Pokemon fan I understand your pain. It’s not like it’s an obscure series, or from a small company. Why is it so hard to stream such a popular anime? I’m surprised The Pokemon Company hasn’t rolled out their own streaming platform yet.
Before diving in to Plex I would highly recommend looking at Jellyfin first also. It’s offers much the same features as Plex but is fully free and open source.
For my own media server I use an old HP Microserer G8 purchased second hand, and upgraded with a Xeon e3-1260L, also sourced cheaply used. It’s small, easy to service and happily runs my Linux disro of choice. I know other people using various SFF PCs, or even repurposed old desktops. For best performance look for a CPU (or GPU) with hardware video encoding support. Otherwise, the rule of thumb for Plex used to be a CPU with at least 2000 Passmark score on cpubenchmark.net per concurrent 1080p stream.
PiVPN is a simple home VPN solution that’s worth exploring.
Is you are interested in smart home/home automation Home Assistant is an open source home automation platform and makes a great Pi project.