Small stylish cheap case with room for hotswap bays? Those are some words which don't really fit together.
Drop the stylish and go for a Silverstone DS380. (yes, that is cheap)
Secondly, look for a motherboard with on board wifi or get a USB wifi adapter because you are inevitably going to need that PCI-e slot empty for more sata ports.
The Gigabyte GA-H170N-WIFI maybe? AC wifi, 6 sata ports, socket 1151, 2 RAM slots. (do M.2 slots work in Unraid? As cache maybe?)
Get a single 8GB RAM stick, that leaves you room to expand.