There's nothing wrong with what you're planning to do - building a game using assets you're paying for is pretty common, and as has been said above that's exactly why the asset marketplace exists. We did it in one of our own projects by paying for a set of character portrait images because we couldn't achieve the art style we wanted with the skills we had available on our team, and thought nothing of it. We'd have needed to spend a lot more to commission and brief an artist, so buying stock assets made much more sense to our project budget.
You can also make use of free assets that people have made (with the correct usage rights), and if you give them a shoutout in your credits or social media then everyone's a winner! Good luck with your project!