I've looked at used briefly but am ok with buying new.
Still, you might want to take another look. You can save significant money and still have a capable system
Some assembly required...
CASE: Supermicro 24 Bay Chassis SAS846TQ Server 4U - - US $399.99 + $64.15 Standard Shipping
https://www.ebay.com/itm/183457141415
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 AND 32GB of memory, but it is really slow CPU, so probably want to replace that:
System Board: SuperMicro X9SRL-F AXT Motherboard w/ I/O, 32GB RAM Intel Xeon E5-2603 1.80GHz - - US $179.99 plus shipping $28.32
https://www.ebay.com/itm/401606434468
Nothing included with this one, so the price is lower, but an option:
Supermicro X9SRL-F Motherboard Socket LGA2011 System Board w/ I/O Shield - - US $189.98
https://www.ebay.com/itm/113216257183
Memory: SAMSUNG 16GB PC3L-12800R DDR3-1600 ECC Registered 1.35V RDIMM - - US $44.95
https://www.ebay.com/itm/302606459277
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.
CPU Fan: Dynatron R27 Side Fan CPU Cooler 3U for Intel Socket LGA2011 (Narrow ILM) - - US $39.59
https://www.ebay.com/itm/401284811045
The CPU that comes in the board above is pretty low speed, so you might find that you need a better one.
You have a lot of options for CPUs to go in this board, but I recently bought one of these for myself:
PassMark score of 13073... If you are wondering... This is the model I use for the NAS I run Plex in.
CPU: Intel Xeon E5-2650 V2 2.6GHz 8 Core 20MB 8GT/s SR1A8 LGA2011 ( Ivy Bridge ) Processor - - US $89.97
https://www.ebay.com/itm/142937685210
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 - - Price: US $189.99
PassMark score of 15796... If you are wondering...
https://www.ebay.com/itm/192637399687
For the drive controller, I would suggest a SAS controller, just to get all the drives on a single controller. It works better that way. One SAS controller like this can run up to 256 drives by use of expander controllers. We can talk more about that when you need more drives but this will get you to eight drives to start.
Drive Controller: SAS PCI-E 3.0 HBA LSI 9207-8i P20 IT Mode for ZFS FreeNAS unRAID - - US $69.55
https://www.ebay.com/itm/162862201664
Drive Cables: Mini SAS to 4-SATA SFF-8087 Multi-Lane Forward Breakout Internal Cable - - US $12.99
https://www.ebay.com/itm/371681252206
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:
Boot drive: 120GB Intel SSD DC S3500 Series HP Model: TK0120GDJXT SSDSC2BB120G4B SATA 6Gb/s - - Approximately US $38.68
https://www.ebay.com/itm/273102509397
Thermal Compound: Noctua NT-H1 Thermal Paste Grease Conductive Compound for CPU/GPU - US $6.95
https://www.ebay.com/itm/302624513215
Just rough math in my head, I think that is all around $1000.... Still, you might need some odd bits, and hard drives, 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 for years to come.