What is the best system config for ue5 performance?

If you cannot afford a Quadro 8000 ($9000) then buying a GTX 3090 with 24 GB is the best and cheapest option. It is cheaper than Titan and much better in performance.

Here are a few of the models you can buy,

https://store.nvidia.com/en-gb/geforce/store/gpu/?page=1&limit=9&locale=en-gb&category=GPU&gpu=RTX%203090

You can also buy a liquid-cooled GPU