Generally the upgrade price from standard to developer edition is just the difference in price between the two. The hosting discount can only be used once, so essentially you're only going to receive that 50% discount for the first purchase.
If you're just changing colors and some text then you likely won't need the actual Photoshop files, but we would recommend you create a
child theme in order to make the changes to ensure you don't lose your modifications when you update the theme. Our
Vantage customer showcase has some good examples of Vantage where the default layout has been changed from which you see on the
Vantage demo. Even with the Photoshop files, you still need some knowledge of css/html in order to actually make use of them, so if you would class yourself as a beginner, then you probably won't find a use for them.