I am looking for my first NAS. I would like to use it as back-up for pc/phone, network drive, Google drive/photo backup, and Plex server.
There will be 4 users, maybe up to 6.
I think ECC RAM is preferable but I do not now how much would be acceptable (as minimum). If 4 GB more the adequate or is 8GB not nearly enough.
I've seen M.2 NVME drives as cache upgrade, is it really necessary and if so. How much would be recommended.
Based on your needs, the Synology DS920+ would likely be a good choice for your first NAS. It offers robust features for backup, file sharing, and media streaming, and can support up to 4-6 users. While the DS923+ with NVMe storage pools and cache would offer better performance, it may be overkill for your current needs, and the DS920+ should be sufficient for most home and small office use cases.
As for ECC RAM, it's generally recommended for NAS devices that will be used for critical data storage or in enterprise environments. However, for your purposes, non-ECC RAM should be sufficient. The DS920+ comes with 4GB of non-ECC RAM, which should be enough for basic usage. If you plan on using the NAS for heavier workloads, such as running multiple applications simultaneously or transcoding large video files, then upgrading to 8GB of RAM would be a good idea.
Regarding M.2 NVMe drives as cache upgrades, they can help to improve performance by reducing latency and accelerating read/write speeds. However, they are not necessary for basic usage and may not provide significant benefits unless you're working with large files or running multiple applications simultaneously. If you do decide to upgrade the cache, a single M.2 NVMe drive with a capacity of 250GB or higher should be sufficient for most use cases. Keep in mind that the cache can also be upgraded at a later time if needed.