Updated Links on: 4 Aug 2018.
These are older components, but still very powerful.
I use similar components in both my 48 bay primary NAS and in my 24 bay backup NAS that runs ESXi with FreeNAS in a VM.
This one comes with a CPU and fan, but it is really slow, so probably want to replace it:
Supermicro X9SRL-F Motherboard LGA2011 System Board w/Intel E5-2650L 0 @ 1.80GHz
https://www.ebay.com/itm/132762499937
Price: US $174.99
Nothing special about this, and the price is higher, but an option:
Supermicro X9SRL-F Motherboard Socket LGA2011 System Board w/ I/O Shield
https://www.ebay.com/itm/401593992194
Price: US $199.99
SAMSUNG 16GB PC3L-12800R DDR3-1600 ECC Registered 1.35V RDIMM...
https://www.ebay.com/itm/302606459277
Price: US $44.95 x 8 = $359.60 for 128GB of RAM !!
note: You could go for the 32GB memory modules, but they are more than twice the price.
If the board above doesn't come with a cooler, or you want a better one, I use this model on two of my systems.
It is only slightly louder than the Noctua cooler I have on my wife's desktop PC.
Dynatron R27 Side Fan CPU Cooler 3U for Intel Socket LGA2011 (Narrow ILM)
https://www.ebay.com/itm/401284811045
Price: US $39.59
You have a lot of options for CPUs to go in this board, but I recently bought one of these for my system:
PassMark score of 13073... If you are wondering... This is the model I use for the NAS I run Plex in.
Intel Xeon E5-2650 V2 2.6GHz 8 Core 20MB 8GT/s SR1A8 LGA2011 ( Ivy Bridge ) Processor
https://www.ebay.com/itm/113166298794
Price: US $91.99
Only it was $30 more when I bought it... It works great. Plenty of resources for all the things I am doing.
If you want more, you can get a 10 core model like this:
Intel Xeon E5-2680 v2 Ten Core 2.8GHz 25MB CPU PROCESSOR LGA2011 SR1A6
PassMark score of 15796... If you are wondering...
https://www.ebay.com/itm/192637399687
Price: US $189.99
For the drive controller, I would suggest a SAS controller, there are 4 SCA ports on the system board in addition to the SATA ports, but only two of the SATA ports are SATA III, so you will still be a little short.
SAS PCI-E 3.0 HBA LSI 9207-8i P20 IT Mode for ZFS FreeNAS unRAID
https://www.ebay.com/itm/162862201664
Price: US $69.55
I would suggest one of these SSDs for the boot drive.
These are used data-center drives, but as a boot drive in FreeNAS, it should last as long as the server, if not longer:
Intel SSD DC S3500 Series 2.5" SATA 6Gb/s, 20nm, MLC 80GB
https://www.ebay.com/itm/183416267073
Price: US $29.99
Just rough math in my head, I think that is all under $1000.... Still, you will need some odd bits like the case and power supply, but it should be simple to get there from here and this should save you a buck or two vs buying new and still do the job.