Before focusing on MAc or PC, on one configuration or another, you need to know what software you are going to use. it will save you the recurring setbacks that we regularly see from people who buy a gamer machine to do 3D.
For used machines, using the search box with the term "used", you will quickly find some discussions with interesting addresses.
I totally agree with Stefbeno, you should already know which software you are going to use, and an important point if you can buy the software and the computer from the same seller, it avoids a lot of compatibility problems.