As a non-native speaker of the English language, I have no idea about purchase/buy and all that, but there's another word that bothers me:
Maybe I'm wrong here, but the way I understood the configuration wizard, launcher etc., there's supposed to be an existing installation of Morrowind on your machine, which's data files you point to, so that openMW can read them and please you with the game experience. I'm fine with expanding that and including some installation help or an alternative way of installation, as for the whole macOS issue.
The first screen, however, says "To start, let's install a game". That just simply doesn't apply to anyone who already has a functioning copy of Morrowind on their machine.
Here's a basic alternative, that I'd gladly see improved: Instead of "To start, let's install a game", just have it say "To start, let's choose a game.". Then, if the user clicks on TES III: Morrowind, a follow-up screen could give the choice between: "Choose an existing installation of The Elder Scrolls III: Morrowind" and "New installation of The Elder Scrolls III: Morrowind". And from there on, the already shown screens could be used.
In any case, thanks for the effort, looks good and really user-friendly