Sir,
Can you use Xeons, yes you can, can you use ECC memory, yes you can. Can you buy old workstations that were decommissioned from a company? Yes you can but if they were decommissioned it's because they are using new stuff that its better. It's the value/perfomance better? Probably depends, of how much you paid.
I deployed exactly the same configuration at my last work but with an Ryzen 1700X back when I was in Portugal with an equivalent Motherboard with proper VRMs with a AMD FirePro V7100 for CAD Working.
The advantages in extra speed in memory are negligible after certain threshold. Plus Memory speed without proper timings its worse that lower speed but tighter timings.
PCIE Gen4 yes is a great deal, no denial, and PCIE Gen5 going to be released next year even more. Although the only hardware released that pings the advantage of PCIE4 currently are some NVME SSDs released this year in Computex.
Current Graphic Cards don't tap the full advantage of PCIE Gen3 16x speed.
For a production environment the use of Water Cooling is basically a risk to be taken. Did you see any company using PCs water cooled, Custom loop builds or AIO even? If it fails is basically production time lost and lost of hardware, specially if the AIO have defects (Enermax AIOs with corrosion, Corsair Pumps failing, etc... If you want I can link recall docs for what I'm saying). A Custom loop should be drained and clean each year, an AIO are good for around 4 years, they are made to be throw away when the pump fails. Never had a Fan in a AirCooler fail in the last 10 years, saw some AIOs fail because of evaporation of the liquid through the rubber tubes:
Finally, tubes are generally made of either FEP or EPDM rubber. The more rigid tubes tend to be FEP, which has excellent reduction of permeation, but less flexibility during installs. Kinking an FEP tube will result in cracking the inner PTFE coating, which results in permeation and poor cooling ability. EPDM tubes have the opposite set of pros and cons: They won’t really get damaged if bent and are more flexible, but it requires an expensive R&D process to get the compound to a point of resisting permeation. Ultimately, all tubes will exhibit the effects of age and will slowly lose fluid to natural processes. It’s just a matter of how long they last. Most CLCs are rated for use in the 4-6 year range, though it’s around years 4-5 that noise begins to get more noticeable. This is because enough of the fluid has permeated the tubes to allow for more air in the line, which gets sucked through the pump and causes gurgling. Users can mitigate this by mounting the tubes down in a vertical CLC install.
https://www.gamersnexus.net/guides/2926-how-liquid-coolers-work-deep-diveAn air cooler with good fans just need a blow with compressed air, and it's good as new. If you want to use water in your own computer for your own production, yes go ahead. But in a deployment of 40 machines no thank you I will not do it.