I known i don't have dove a good gob to specify the goal, nor balancing the different types of upgrades, and i'm truly sorry for that... I hope to improve in my next games...
In short, the goal is to reach the overflow (I assumed the title was enough to explain). See others comment below if you want a more accurate explanation.
For the upgrades, unfortunately, I can't do anything to solve the problem. If you want a greater challenge, try to complete the game without counting only on the typing speed upgrade.