Buying a new machine for Hashcat

[undeath]

Unless you need the CPU for other related means, eg wordlist processing, or you anticipate you need to crack GPU-hard algorithms like scrypt you don't need a fast CPU. If you can get it really cheap it won't hurt of course.

hashcat assumes you have at least 2GB RAM per ocl device. 16GB for four GPUs is okay. GTX1080Ti FE is a good decision.