In case anyone is wondering, here's what my current build plans look like. I still need to buy fans and some Molex to fan 4pin power cables, but I'm holding off on those until I get the case.
Fractal 804 case ($142.91) https://www.amazon.com/gp/product/B00K6OVG0I
(10) Seagate Exos 14 TB drive ($2090) https://www.amazon.com/gp/product/B07T63FDJQ
Intel RAID Expander Card (RES2SV240) ($155) https://www.amazon.com/dp/B0042NLUVE
Power Supply ($94.50) https://www.amazon.com/gp/product/B08PDV45F8
(3) Mini SAS to SATA Cable ($41.97) https://www.amazon.com/gp/product/B012BPLYJC
Sata power cable splitters ($6.99) https://www.amazon.com/gp/product/B07T356R21
Sata 90o angle adapter ($6) https://www.amazon.com/gp/product/B07P5N714P
Total cost: $2442.87
RAIDZ1 and you will have a significant chance to lose all of that data on a rebuild and have to restore from backup. If you don't have backups, even worse.
RAIDZ2 with 8x6 tb drives should give you 36 tb of storage. Up the drives, get a proper interface card and use solely that because that motherboard only supports 6 sata ports alone and it's over 2 controllers. You wouldn't want to have your drives spread over two different chipsets of sata controllers in the same pool would you?
http://www.amazon.com/Intel-RAID-Expander-Card-RES2SV240/dp/B0042NLUVE - something like that run it with RAID turned off, and update firmware I think is required to make it function with FreeNAS from what I have read. This gives you 24 drives worth of ports.
They have these used on ebay sometimes as well. Many people confirms this works with FreeNAS, but I have no personal experience with this specific card.
As others said you need unbuffered ECC RAM for that motherboard (I have the same one).
Watercooling totally unnecessary and potentially a worse solution than a regular cooler because the pumps die on watercoolers more frequently than fans and you can always replace fans on the heatsinks. Heatsinks never fail.
7200 reds as well as said elsewhere, paying extra for the 7200s when 5400s are sufficient. 5400's are more than fast enough.