its the correct sub forum, and there are dozens or hundreds of threads asking the same thing about hardware, you might want to do some further searching for plenty of various opinions.
an intel 7000 series with integrated 600 series gpu is all you really need.
find the best performance per $ going from there.
also, plex gpu transcoding requries plex pass, otherwise you are limited to CPU transcoding only.
if you are not expecting to do any 4k/hevc transcoding, go for the biggest passmark cpu you can afford.
if you want to do 4k transcoding, then you will need a plex pass, or an even bigger cpu.
for 4k info see @ Plex, 4k, transcoding, and you
