Again, performance needs depend on desired lower end platform.
Re texturing:
Whatever works with the smallest possible file size (in pixels, as power of 2) is always best.
You can do pretty much anything, so you should study up or review mesh material methods depending on what ypur gameplay will need to accomodate for.
I have no idea. Check the heights of the tallest points - even than 600m isn’t that high of an elevation.