I'm thinking about purchasing Warhammer Quest, but I have a question.

Does the game have microtransactions? They are shown in the trailer at 0:54.

The GOG version does not have microtransactions and comes with the additional content already active. See comments from here: https://www.gog.com/forum/warhammer_quest/warhammer_quest_deluxe_additional_content

That said, I've found that you may not want to use the bonus weapons at first, or at least not all, as the game is too easy with them in play early on and using them may spoil it. Further along I decided to keep the weapons and drop down to only 3 heroes and it's been just about perfect.