Depends how you define inexpensive I guess but I'm using ASRock boards that support unbuffered ECC with my Ryzen 3000s. For a server build the
X470D4U and X570D4U are popular choices but they're much more expensive than desktop boards and the ECC error reporting is iffy (worth it for IPMI though in my opinion).
Last I looked the Intel chips that supported ECC were server-only and rather more expensive than the AMD equivalent but that was some time ago now.
IIRC there's a fair few people using intel quicksync transcoding with plex so it might be pertinent to establish if you're going to be using this functionality and whether it'll work under freenas and friends. Other people use an nvidia GPU for transcoding purposes, and of course software transcoding is likely also an option.