Adding a GPU to the Mac Mini isn’t going to be cheap… you’d need to get an external GPU enclosure such as this: BizonBox 2, but, then you’re looking at least $500~$600… for just the box, NO GPU included, building a Windows machine for a workstation is a lot cheaper.
Not that I really suggest adding a GPU to a Mac Mini, might as well build something else. Just say the GPU is the weakest link.