18 TB disks are still the sweet spot price wise it seems. So I would go for that as many as you can afford. The rest depends on what you actually want to do with the NAS. IMHO, you don't need insane amounts of cache, if the NAS shall just serve some media files. The HDDs itself will be plenty of fine. Just one SSD for the system installation should be all you need, IMHO.
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Disregard the advice about SSDs. Start with 1 NVMe for boot and only add more if you have a need. Either for running VMs or if additional cache is actually needed.
Check Out patriot p300 nvme, 128g boot drive for like 10€. I have two of them, one running flawlessly almost 2 years
Thanks :)
As it was mentioned, there is no real need for SSD caching unless it is required by your workload. In any case, you should be able to configure caching later if the performance of HDDs wouldn't satisfy your needs.