It really depends on what you need.

Plex relies on the client device to provide the necessary compatibility to play thing, otherwise the server needs to transcode it into something that is playable.

Which means that when you curate your library in a way that is completely compatible with your clients, you can run a Plex server on a raspberry pi. If you don't, and you need transcoding then you also need the hardware to do so.

This also depends on what needs to be transcoding, a 4K video will require a lot more performance than a 1080p Transcode.

Then, you need to distinguish between the different types of transcoding, when you have a Plex Pass, you can use a GPU for the transcoding, otherwise your CPU needs to do that.

In the last 1-1.5 years, the beelink mini pc with the N100 CPU was really popular because it was small, fairly cheap and had a iGPU in it that could even handle a couple 4K transcodes.

So, the first thing I would do is check what your requirements are. Do you have Plex Pass to be able to utilise hardware transcoding? Maybe also look in other players like Plezy for getting better compatibility than the official Plex app.