Welcome,
Since your requirements are not enterprise level… any drives can be used for your application. What’s more important is selecting drives which share similar characteristics (example) rotational speed, cache size, etc. Disk size is less of an issue however, the overall size of the array depends on this, and on the RAID Level used.
Take your time selecting the NAS box and do a little research. Ensure it supports the protocals needed for your network and intended application. I’d also recommend looking into it’s chipset and amount of installed memory. This can directly effect sustained throughput regardless of the speed of the ethernet port 10/100/1000. Most typicaly operate at 100 so a GIGABIT network means nothing.
