Ok, so I understand you're looking for more storage space, more consolidation.
Of note, this isn't a goal: "My main goal was to have this second pc running more in the background." It is a proposed solution, which can deflect from determining the real goals and requirements.
I mention this, because, the solution above drags in additional unforeseen work and issues to resolve, such as:
- the system will then need a backup solution
- the system may use more power than other solutions
- the system will require additional, ongoing OS maintenance work
- the system may require additional infrastructure (network plumbing / gear, power gear, temporary monitor, keyboard, mouse, etc.)
- etc...
I'm my opinion, unless you have a very large media collection, and several systems using the media, for most situations, a single system with enough storage is usually sufficient and more economical in terms of time and cost.
For adding disk storage, either a self-contained NAS or a disk array with large capacity, quiet, energy efficient drives is the way to go. Replace inefficient, smaller capacity disks with models that are more appropriate to the requirements.
Donate (or sell) the old PC so that it will be used as a cheap PC.
My 2 cents, for what its worth.