I tend to get Despair at lvl3, because with Q stunning, and spamming E, you can punish the enemy for attempting a gank. Also, you should try to embed skills/items into the guide, for convenience.

Nice start for a guide! Take note that cooldown reduction can allow you to max tantrum and despair 1st, tantrum is good for last hitting and is somewhat spammable while despair helps alot in team fights also. Amumu is pretty fragile for a tank, but he got despair as an anti-tank skill to make up for it, to get other tanks as well. Heal is useful for a summoner's spell as it allows you to stay in fights longer and helps early game as well. Note : Evasion is useless to amumu, seeing he has no dodge rate w/o ninja tabi. 102% of 12 dodge rate doesn't really help. Aiming to max a single skill first after adding all skills a little is usually better, lvl 4 means having all skills but ulti. Oh, and you can also add a situational item list if you want :)