If it’s just for storage, you can use a Pentium dual core from the 6th or 7th gen Intel CPUs just fine. My NAS runs a Pentium G4560T (T suffix = 35W model), which is based on Kaby Lake, or 7th gen. You can pick up one of these CPUs for stupid cheap, I think they go for maybe $5 or $10 here in the US. The NAS has not complained at all. That said, I am going to upgrade it to an i5-6500, but only because the i5 has vPro capabilities, which I use on my other machines.