The 16G version is 679$
…and there’s no slot for a 10Gbit card (which probably is more than what that SoC can manage)
16Gbyte is official tops for the N-series, not sure if you want to go out of spec on that if your data is important. They’re really vague about the hardware in general.
On the R5 you remove the side panel and there are trays not sure why you’d risk data because of hot-swap but oh well…
If you’re looking at the same platform the Odroid H4 Plus seems like a much better option.
Get the board + M.2 2×2 Card – ODROID + RAM
Get one of these controllers: Short Review: Edging ASMedia 1166 PCIe Gen3 x2 to 6 x SATA HBA Chipset. It doesn't suck 👍
Grab something like the Sagittarius NAS case, mount the Odroid H4 Plus on standoffs and reuse your old PSU.
That’ll be around half the price of the Zimacube but with more storage.