Samusng smart tv might work for you since, it has mots of apps including the Plex app. However, I do not think it would work over wifi well. I've treid to stream DVD rips over it, and it was very choppy. I've also tried streaming, over ethernet, 1080p bluray rips with uncompressed sound, and tv could not handle it.
My suggestion would be to use intel NUC or Gigabyte Brix systems. They are tiny and designed to sit behind the tv, I think brix even has mounting bracket so you can attach it to backside of tv.
If you dead set on not using htpc, you might also want to check out Samsung's smart tv evolution kit, which supposed to improve build in smart tv functions.