Two questions: will the Steam version be the same as this one or will they add something extra? And second, if I buy the game and later another update comes out, will I have to buy it again?
Steam version will have a lot of extra stuff, but itch version will be updated as well once it drops. No you won't have to buy the game again, however unfortunately I will not be able to give steam keys to Itch customers because it would be a breach of Steam's TOS.
