cpu - Dual xeon wil help you more the the i9 in most of the cpu based computation sections. There are a few places where it single thread only, but most of the time it keeps all 16 cores in my box (see below) busy. Find some older server gear with enough pci-e x16 slots and you should get a good platform to build on.
Also you may want to consider just renting cloud time rather than investing in hardware that you only work hard a few hours a week. AWS has some nice instances for the (p3) which scale for power (and cost) to some really impressive configurations (p3.16xlarge - 64 intel core, 430gb ram, 8x tesla v100 gpu etc)