Depends on the licensing constraints of who hired you.
You can’t really re-sell someone else’s art/work to another developer.
That aside, alien landscapes are kinda simple.
Most are barren and nearly textureless or monochrome.
One of your samples has some vegetation which could be a bit of a grind to get going, but doesn’t have to be.
As you asked.
stay away from .26 unless the client commissioned it. Because performance wise it’s possibly the worse release in the last 1 year and and a half (Perhaps a LOT longer than that).
Basically consider .26 as a preview version. there’s so many things wrong with it, even the devs don’t bother keeping a list XP
Sky atmosphere still works in .25.
but realistically stay away from that too? Its basically overcomplicating a simple thing and you can make stuff look a lot nicer with much better performance by using the old BP skysphere system - when on planet.
It really all depends on what you acually need this for. A game vs a cinematic for example.
Realistically, don’t know about the other guys but whenever there’s a jam we make a heck of a lot more stuff than just an environment in less than 7 days…
You can totally complete this task even without knowing what a Normal map is today 