@retrospective

Why Xeon? It is most performance/cost inefficient option.

For price slightly more than of cheapest shitty Mac Pro you can make pair of top 16-core AMD systems in Thermaltake Core W200 CA interconnected by Infiniband. Later just upgrade to pair of 32 (current) or 64 (next gen) core CPUs, all else will remain.

If you so like can install ECC RAM in TR4 board, it is supported.

If you are not in corporation (or big editing team) or running server 24/7 I see no point in EEC memory.

@4CardsMan

OK, lets make standard ATX system that similar, but more powerful and really upgradable

• $199 GIGABYTE Radeon RX 580 DirectX 12 GV-RX580GAMING-8GD 8GB 256-Bit GDDR5 PCI Express 3.0
• $170 32GB DDR4 - 2x GeIL SUPER LUCE RGB SYNC AMD Edition 16GB (2 x 8GB) 288-Pin DDR4 SDRAM DDR4 3200 (PC4 25600) Desktop
• $499 AMD 1st Gen RYZEN Threadripper 1950X 16-Core / 32 Threads 3.4 GHz Socket sTR4 180W YD195XA8AEWOF Desktop
• $145 Thermaltake Core X71 Tempered Glass Edition
• $139 EVGA SuperNOVA 1000 PQ, 80+ PLATINUM 1000W, Modular, EVGA ECO Mode, 10 Year Warranty, Power Supply
• $80 4xPhanteks PH-F200SP_BK 200mm Case Fan
• $90 be quiet! Dark Rock Pro TR4
• $159 SAMSUNG 970 PRO M.2 2280 512GB PCIe Gen3. X4, NVMe 1.3 64L V-NAND 2-bit MLC Internal
• $349 ASRock Fatal1ty X399 Professional Gaming

Total - $1830, can add pair of cheaper 1TB NVMe SSDs to make it $2000.

All prices US, Newegg or B&H.

Such system will be whisper quiet, extremely upgradable (up to 420+420+360 radiators), can add second PSU, MB really bear 4x GPUs. Does not have Thundebolt by default (can add PCIe card if you so like), but has USB 3.1 Gen 2 and 10Bit network (+2 other network cards).

Also Macbook Air with handles and in three colours!