Over the years I've built four TrueNAS Core ZFS servers, two of them in HP ML350p Gen8 server chassis with ECC RAM, the others with standard PC components and non-ECC RAM. Error Corrected RAM is not 100% essential for TrueNAS, but it does reduce the chance of data corruption going unnoticed. You can fit ECC RAM in some normal Intel/AMD motherboards, but you're more likely to get ECC support with a dedicated server motherboard.
You don't need a fancy motherboard with multiple SATA ports for a NAS. Instead, consider an LSI SAS (Serial Attached SCSI) HBA (Host Bus Adapter) controller such as the LSI 9211-8i. These cards can be picked up second hand on eBay for under 25 dollars/euros/pounds. I avoid buying brand new so-called "LSI" cards from China because they're usually fake and not the genuine article. Get your cards from local suppliers specialising in breaking up old servers.
Suitable LSI SAS HBA cards typically come with one or two SFF-8087 ports, each of which can control four hard disks. To use an LSI 9211-8i with eight hard disks, you'll need a couple of SFF-8087 to SATA breakout cables. SAS controllers can be connected to ordinary SATA drives, or you can use more professional 6Gb/s SAS2 drives instead. This leaves the SATA ports on your motherboard free for other drives that are not part of the array, e.g. an SSD for the TrueNAS operating system.
The LSI 9211-8i comes with a PCI Express x8 interface, but will work quite happily when plugged into PCIe x4 or PCIe x16 slot. If your PCIe slot has only 4 lanes, maximum bandwidth will be halved, but this isn't usually noticeable with "older" hard disks. If you create an array using modern SATA hard drives of at least 8TB capacity and 250MB/s transfer rate, I'd recommend sticking with PCi x8. For 12Gb/s SAS3 drives I'd use a newer more sophisticated HBA card than the 9211, e.g. 9300 series.
If you're planning to use TrueNAS Core or UNRAID as your operating system, it's vital you buy an HBA card that's been flashed with an IT (Initiator Target) BIOS, that passes commands through to the hard disks unaltered. TrueNAS does not work properly with a standard IR (RAID) BIOS, which "hides" the hard disks from the ZFS operating system. Although it's possible to re-flash an HBA card from IR mode over to IT mode, it's not easy for novice users. If you're going to use TrueNAS or UNRAID, buy an IT-flashed card for an easy life. Second hand IR HBA cards are often much cheaper than IT HBA cards, despite the only difference being the BIOS.
I built a basic ZFS system for a pittance using a second hand Fractal Design R4 case with eight 3.5" drive trays (£15 on eBay), an ancient dual core Athlon APU and mobo (£5), eight 4TB Enterprise level ex-server drives (£10 each), 16GB of non-ECC RAM, a 64GB SATA SSD for TrueNAS and a good quality PSU with 10 SATA power connectors.
With TrueNAS set up for RAID Z2 (equivalent to RAID 6), you lose two drives' worth of capacity from the array, so 8 x 4TB drives equates to 6 x 4TB usable capacity. I've only had one drive go down and that was a 4TB Toshiba N300 purchased brand new in 2018 with less than 6 days total use. My 4TB ex-server drives have over 1,400 days use, but I keep multiple backups of data, so it doesn't matter if one system fails catastrophically.
For at least 50TB of usable capacity and assuming RAID Z2 or RAID 6, you could run eight 10TB drives, which would give you 60TB usable space. Eight 8TB drives would give 48TB usable capacity.
One advantage of using TrueNAS Core and the ZFS filing system is that if your computer hardware fails, you can transfer the entire hard disk array to a totally different computer, switch it on and with a few tweaks (or load a previously saved configuration file) and you continue as if nothing happened. Try this with a proprietary RAID system and you'll probably have to buy indentical hardware to restore normal operation.
When I upgraded the drives in an HP ML350P Gen 8 TrueNAS server, I transferred the eight 2TB SAS drives over to a desktop PC and the system booted up normally after restoring the configuration setup. It didn't matter that I switched from an Intel Xeon with ECC RAM to an AMD APU with non-ECC RAM. TrueNAS is not fussed about hardware, provided you have at least 16GB of RAM and a suitable drive controller. For a 50TB system, I'd suggest fitting at least 32GB RAM with TrueNAS.
If you want to save money, consider a DIY TrueNAS or UNRAID system. If you want an easy life, buy an 8-bay Synology or QNAP chassis. It's relatively easy to build a DIY TrueNAS server, but you could spend ages learning to configure the system. It takes a fair degree of patience and online help guides before you can get TrueNAS going. That's where the Synology/QNAP UI wins hands down for ease of use. N.B. I don't have a Synology or QNAP system.
A good source of advice can be found on the 'TrueNas' forum and 'Serve The Home' forum.
Caution:
If you end up building a TrueNAS server, do NOT use SMR (Shingle Magnetic Recording) drives. Instead use CMR (also known as PMR) drives. Back in 2020, the FreeNAS community discovered Western Digital Red drives advertised for use in NAS systems were using "incompatible" SMR techology. There was no mention of the fact Red NAS drives were using SMR in the data sheets on the WD web site. TrueNAS/FreeNAS is particularly intolerant of SMR and when you replace a failed drive in an SMR array, it can take TrueNAS several days to resilver instead of a few hours. Check very carefully before buying drives for TrueNAS and make sure they're CMR/PMR and not SMR.
