fun idea. have the text checked by a native speaker. if you make some edutainment it should teach propper spelling and grammar
also you should have a look at the capping of the upgrades, since you end up with gears that have no word.
and i would find it more challenging if you would have to type the words from left to right without a mistake. like if you mistype at the third word you would have to start over. that would give a strong incentive to make less mistakes.