AFAIK, Megascans are free only if you use them in Unreal engine, otherwise it's paid product, so keep this in mind. As for the texture resolution, it depends on many things, like final resolution of your rendering, how close the subject is to the camera, how much tiling, if any, does texture have, etc. Usually you go with the highest resolution that your PC can handle in particular scene to be on the safe side. From what i know, Corona 9 should be big improvement in this regard, since it will handle some of those things automatically (only if you're using Corona bitmap).
As for the topic relocation, i can do this if you want, but if you don't plan to work on the images in the first message, then current board is perfectly fine.