You’re asking for a lot of different things that don’t align, so instead of trying to guess what you need, let me just throw a few things out there:

• if you want transcoding that isn’t constantly tied to GPU, you want an AMD chip of some sort. If you’re trying to be efficient, and APU model.
• you probably want to just merge all your data into a single disk array. Running multiple on one system is pointless and inefficient.
• separate your CPU needs from storage. Like you said, maybe you just need an ultra low power NAS, and then different machine to handle compute.
• skip SSD for any network storage. Yes it’s more power efficient, but if you’re talking bulk storage, you’re wasting resources for smaller volumes where it won’t matter of transferred over the network. SSD is good for local-only for the most part.