You might want to take a stock of the planned things (VM Host, FreeNas, what else) and define the components you need for it.
This might include a HBA for FreeNas (x8), maybe a NVMe Card for slog (x4), maybe a 10G or faster network card (x8).
If you need anything else (for example I run vSan so I need two additional x4 slots) note it down.
These parts must be either on the board (-C variants for HBA, -T variants for 10G) or pluggable via slot.
X9SRL should be fine for most of it unless you need x16 slots (GPU, newer Mellanox cards) or bifurcation & boot from nvme (not sure whether x9 boards support these).
Cpu wise a single CPU should be fine for your use case, you could get the venerable 2670v1 for cheap or some of the high end ones 2693/6/9 depending on preference for cor count or single thread speed.. (Of course the e5-16xx series will work too for even higher single thread/less core options)
Single CPU board will be less bulky too so should fit your desired case without issues (which might not be the case with a DP board).
So all in all - sounds good