I’d recommend taking a close look at the EPYC 3000 series SoC. I’m aware of 2 manufacturers offering boards: Gigabyte and Asrock Rack.
I’m actually in the process of purchasing the Gigabyte MJ11-EC0 (link) which will cost me around 500 euro (say CAD 650-ish). It’ll do what I want (essentially a NAS) but it rather limited in terms of external connectivity (no 10Gb LAN, single PCIe slot) But it’s a small board that fits pretty much anywhere.
On the opposite side of the spectrum I found the Asrock Rack EPYC3451D4U-2L2T2O8R (link) More connectivity overall (dual PCIe slots, dual 10Gb NIC, etc) and a slightly bigger board that’ll still fit in most cases. But I also found a 2k USD price tag on Newegg recently for that board 
There are other options: I recently purchased a Asrock Rack X470D4U (link) and paired it with a Ryzen 7 1700 (non-X) and 2x 16GB non-ECC RAM sticks. Neat system, although the thread for it is rather long 
Most notable nuisance is the distance between the CPU socket and the nearest RAM slot, which influences the choice of cooler you can use. I’ve solved it by not populating these slots, for now, but also a smaller cooler for older AMD systems (IIRC mine is for AM3 CPU’s, doesn’t really matter) For a 5950X, I’d recommend the X570 board Asrock Rack offers, the X470 board may struggle power-wise.
-
All Ryzen CPU’s are ECC capable, but not officially supported. YMMV!
-
Look at the nVidia professional cards, like the A400 or similar. probably not the cheapest solution!
-
A 100W PSU is overkill, unless you include more then a single GPU to the system, 750-850W suffices on a single GPU but for a no-GPU system 400-500W is plenty, even with a full HDD RAID6.
-
Noctua has very capable fans and undoubtedly equally capable coolers, but don’t underestimate the AMD stock coolers. Having said that, your choice of cooler depends as much on the case you’re building in as the performance/heat production of the CPU and optional GPU(s).
HTH!