Are you purchasing a PSU for this build as well? If so I figure you'll probably spend ~$100 for a PSU as well. If you can go rackmount the 3U supermicro CSE836 series can be had for cheap on ebay and typically have 12 bays for ~$300. That is an additional $100 for more drive bays, redundant PSUs, and a likely better built chassis than the one you posted. In fact, here is one for under $300 which even has a board (though it is quite old and perhaps you'll want to not use it). Here is another with a direct-attach backplane that can be used with 1 HBA if your motherboard has plenty of SATA ports; else you can pick up 1 cheap expander.
Honestly i don't think i want to mess with used psus on a $6000 system lol. But i for 100 extra it seems ok for the extra disk capacity. Though my understanding is that freenas doesn't allow for expanding a raid one disk at a time. You essentially have to swap out the entire array to expand it.
Plus i would prefer to stay 4u just for the sake of airflow and being able to get a decent heat sink for the cpu.