Hi, from the marketplace guidelines:
2.1.d Content must consist primarily of original work and differ from the
result of public tutorials.2.1.e Sample content from Epic Games must not be an integral part of your
product, but used for display or
example purposes only.
so sure Megascans is free to use in unreal engine, but I’m not sure if your described project will be accepted in the marketplace due to the guidelines, because as far as I understand it the Megascans Assets would be an integral part of your project (best to wait for someone from epic games to answer).