To design software, you're better off with Windows as almost every software written will be compatible with it. For the minimum requirements, it depends of the software you're going to use. The bigger the geometry of the models you're planning to create, the best hardware you're going to need. But as you won't be placing objects on the map as a level designer, your computer won.'t need to see more than one object at the time so it'll be less performance-hungry (still, depending of the geometry of the created characters)..