Hi,
I am currently using this setup with great results on Resolve. Don't use Fusion, but if it can handle fairly complex AE projects, I assume it would handle Fusion too.
Mac Pro 2012 with 2 x Xeon X5690, 64 GB RAM, Nvidia GTX 980, and an assortment of SSDs and WD Blacks in RAID 1 and 0. Also using a PCIe card for Esata and USB3.
So you don't really need the latest and greatest $2000 CPU and $1000 GPU, just an optimized build