NAS with a NUC

Hey guys. Been reading about the NAS that synology offers and I would like to ask for some advice.

The primary use I need the NAS is for it to be a file share as plex runs on the NUC. But I would like it to be able to transfer data quickly, provide backup solutions. And I am thinking of potentially expanding it with the dx517

I am considering the 420j as its cheap enough, has 4 bays and can provide the basic functions. But I just want to know if the 420j will be too slow or limiting and should I pay the extra for the 420 plus?

Thanks for your help.