Sketchup graphics card for rendering
It’s not clear from your post which version of Mac Mini you have, and that affects the answer. In general, there is no space inside a Mini to add a graphics card if the built-in graphics aren’t powerful enough; you have to resort to an eGPU. And from what I have read, the new M1 Mini does not support an eGPU. So unless you bought an old model Mini, you are stuck with what came in it. However, It isn’t clear yet how the M1 graphics will perform for rendering once they are supported natively by the renderer.
Adding an eGPU can drive the cost of the total system up close to that of an iMac. Also, Apple doesn’t support nVidia graphics cards since about 2013 or so. You will need to look into Radeon. Some renderers such as VRay will do hardware accelerated rendering only on nVIdia.
As far as choosing a renderer, that will also depend on what version of SketchUp you are using. Your profile says you are using SketchUp 2014 Free (web) which is impossible.
I have a mac moni 2018 and sketchup pro 2018
Please correct your profile. That information helps us help you.
If you have a mac mini, you do not have a choice. Your graphics card is in the box and cannot be changed.
As for renderers, search the forum… there have been many, many threads on this topic.