Hi. It's a one time Payment. The game'll have some paid DLCs in the future, too.
Overall, you can see it from this pic in the game page. Anything that not marked as DLC will be included in the game and it's 1 time payment so you don't have to worry about paying for new updates. All updates are free!
There're some exclusive artworks ingame for people who's getting the game asap. There's no way to receive them if missed.
