Hi,
After much deliberation and reading I found a reputable Ebay seller and got a newer 2019 iMac (19,1) to modify. They are much cheaper now.
I put in an Aura X2 1TB for the OS, an OWC Mercury Extreme Pro 6G 2TB as a scratch drive, and 40TB of DDR4 2667 Mhz RAM. I also use a large OWC RAID 10 box with SoftRAID and can work directly off of that via the Lightning 3 connection.
It has performed remarkably well, fast even with multiple media programs running, no issues. The only files that sometimes are little slow are extremely large medium format images, to be expected.