Maxing out all upgrades means you have to acquire all skills. To acquire a skill you need to spend Skill Points on it. The amount of Skill Points you need for each skills differs, but in total you will need 69 Skill Points. Every time you level up, you will get 2 Skill Points, but you only get Skill Point from level 5 on. After one playthrough you will be at level 31-35 approximately and to get 69 Skill Points you need to get to level 39. You should get the last few levels while working on the Samurai achievement - if not, replay Level 5 and throw people off the sides to quickly gain experience.