Walkout basements add a nice feature especially when the basement is finished, it is definitely worth doing it. In your situation you have a nice slope away from the house so doing a nice patio and providing sufficient natural drainage is not a problem.

As the rental goes, if the area is zoned to have a rental unit in a single-family neighborhood and in a bi-level house, you can easily rent it out, but you would need to provide kitchen, bathroom, egress, etc to be within building and fire code compliances and all zoning requirements for a rental.

That being said, not sure if this deck was done with a permit, but the footings you have there seem to be undersized for a 6 X 6 post.

Good luck