Hey, it's not a dumb question. Once you buy a game you get access to any files uploaded to that project. You don't need to re-buy a game if they add new files, or change the price later.
There is one exception to this rule, some projects can individually price uploaded files, and then you would only get access to those files if your purchase price high enough. Most developers do not use this feature though, you can tell if it's being used if you see a price next to the file when on the buy screen.
Regarding giving more money later, at the moment you can't add money to an existing purchase, but it's something we'd like to explore in the future. You can, however, buy a game as many times as you like.