So long as you don’t make those textures available to the public (i.e. through releasing your model with the textures you used from the marketplace), I don’t see why not.
https://publish.unrealengine.com/faq - Business terms say you may modify the content and use them in your game that may be shipped finally. But the creator/owner of the asset-set from the market-place should still be able to sell the asset-set to other developers as well - so you may not redistribute it just in source-form (like *.png, *.wav, *.cpp or whatever) as stand-alone libary to other developers in some other market-place (even if you modified it a bit or a lot). However it does not limit you to create your game.
As for the actual workflow I’ve got the maps and a couple of the models imported into blender and I’m messing around combining and editing to make what I need.