I had to get off mobile so I can put out something more long winded. My specific build is one of cost vs performance and having a clear upgrade path in mind. Let's start with what I originally bought.
Base Computer Lenovo TS140 w/ i3-4130 & 4GB DDR3
This computer served me well for a cheap personal computer always with the intention of making it a server. It was relatively cheap at the time ($230 off Amazon), supported ECC RAM, and had 5 onboard SATA ports along with enough PCIE connectors. I did need to get a customer connector to hook up a generic PSU to the motherboard, but that was cheap and widely available.
This is what the machine currently is
Intel Xeon E3-1245 v3 ($165 - ebay used)
32GB ECC DDR3 RAM (ebay as well)
5 x 4GB Seagate NAS (ironwolf) drives in Raid Z1 (media storage)
5 x 4GB WD Red drives in Raid Z1 (media overflow / archive)
2 x 4GB Seagate NAS drives in Mirror (network storage)
Zalman ATX Mid Tower Case - Black MS800
3 x 3x5.25 to 4x3.5 similar to but not this model https://www.amazon.com/Rosewill-RSV-3-5-Inch-Cage-RASA-11001/dp/B005FHHOXE/ref=sr_1_24
IBM ServeRAID M1015 crossflashed to LSI9211-8i IT mode
[ https://www.servethehome.com/ibm-serveraid-m1015-part-4/ ]
With all that out of the way, there are a few caveats with my rig, so I don't recommend recreating it exactly.
#1 There is an issue with the TS140 motherboard and a FreeNAS 11.2 bootloader bug causing it to freeze on boot. I'd recommend a supermicro server PCB if you can swing it. You'll often get more onboard SATA so you may be able to ditch the LSI card I have for more SATA io.
#2 The upgrade path for the TS140 only has support for 32GB of RAM. FreeNAS recommends running 1GB of RAM for every 1TB of storage for optimal use. Also, don't run FreeNAS without ECC RAM, you'll be asking for trouble.
#3 You may want to use larger hard drives, but I keep with 4TB because of the price per GB you get from these drives. Watch when these go on sale at Microcenter or Amazon and you can get them for around $100 a piece.
#4 Crossflashing the cheaper IBM card to the more expensive LSI card is a great hack, but it was the biggest pain in the ass. Some motherboards just don't fully support flashing over PCIE. Some need to boot into BIOS, others needed UEFI and in the end I almost gave up. It took many different trials on a couple different mobos to get the Goldilocks config.
#5 I don't remember the power supply used, but you need a decent one for all those mechanical drives to spin up on boot. I used https://outervision.com/power-supply-calculator to make sure I had enough juice.