My advice is don't do that. You are talking about a home-brew NAS which would be limited on a 2017 iMac to gigabit ethernet, which is about 100 megabytes/sec.
You can get various sizes of an OWC Thunderbay 4 which is vastly faster:
https://eshop.macsales.com/shop/Thunderbolt/External-Drive/OWC/ThunderBay-4
These will be much more compact and quieter than making your own RAID box. For RAID-0 you don't need any external software since macOS supports that. For RAID-5, etc. you would need SoftRAID, which is a superb product:
https://www.softraid.com/
In general I've switched from RAID-5 to RAID-0 (for media storage) since it must be backed up anyway and doesn't change that often. While SoftRAID produces excellent RAID-5 write performance (better than hardware RAID systems I've tested), it's still not as fast as RAID-0 and entails a 25% space penalty. I use several Thunderbay 4 units in RAID-0 and use them in pairs, a primary and a backup. They are very fast. It is better to over-estimate your storage requirements and plan on maintaining about 20-30% free drive space.
If you want SSD, one of the least expensive ways is to use a Thunderbay 4 Mini and add your own SSD cards:
https://eshop.macsales.com/item/OWC/TB4MJB0GB/
You could make a 4TB 4-drive RAID-0 SSD for about $1400, depending on what brand SSDs. An 8TB version would be about $2500-$3000 or so. I use an 8TB version with 4 x 2TB Samsung EVO 850s, and it does about 1150 MB/sec write, 1230 MB/sec read. However SSD performance (and cost) is definitely not needed for most video editing.
You should do a careful, all-encompassing evaluation of your workflow based on shooting time, bit rate, camera and editing codec, extra space for any transcoded files, temp files, cache files, etc. If that produces a storage number where you can afford SSD, then that's OK. However in many cases when shooting uncompressed 4k or RAW, the cost of SSD storage is not affordable.
An older rule of thumb for scripted narratives is they'll use a 10:1 shooting ratio. IOW if the final product is 1 hr there will be 10 hr of material shot. Since the digital age, shooting ratios are much higher, 30:1, 100:1 or more:
http://nofilmschool.com/2016/03/shooting-ratios-mad-max-fury-road-primer-hitchcock
Documentaries will generally have a higher ratio than scripted narratives. 100:1 is not uncommon.
Your shooting ratio is a starting point to determine your storage needs. If the final product is 1 hr, your shooting ratio is 50:1, and the cameras product 8-bit UHD 4k at 100 megabit/sec, that is 12.5 MB/sec * 3600 sec/hr * 50 hr = 2.25 TB. At first that doesn't seem like much but if you must transcode to ProRes, that becomes 6x larger or 13.5 TB. Add in space for scratch files, render files, etc. and it can easily be 15 or more TB.
The same material shot by a RED RAVEN RAVEN at 4.5K 120 fps using 15:1 REDCODE compression equates to about 120 megabytes/sec, and the storage requirement would be 10x larger than 4k H264 or about 22.5 TB, not including transcodes, proxies, etc.