1. EARLY PROTECTION so he can use his E knowing that he won't die at all, that's why those CORE ITEMS: you get tenacity and MR on those Mercury Treads, you get the tankyness/damage from the thornmail+E+W plus the sustain and life of cinderhulk, it's a perfect early core for hedgehog. ;
2. Rammus is a tank for whom people build AD, but they don't understand that most of Rammus's damage is MAGIC. Thornmail, cinderhulk, W, Q, R? All magic. So you probably should think about getting MAGIC PENETRATION;
3. Well, like any other champion, you'll need damage, and how you're going to get a good burst out of a offtank? Well, well. I got your answer. A good ol' SHEEN, and 'why?' you ask. WELL. When you use your E, which damage do you deal? None. When you activate your W and Q, is it enough to murder a carry? Nope. But everytime you use an ability you'll have a little burst that will BOOST YOUR DAMAGE.
4. Dude, you got an E that can force people to attack you and a W that will make you almost invincible, but like any other tank, you'll need BONUS HEALTH WITH DAMAGE, so you can take more damage and use better your thornmail+W damage, and damage because you'll need something to boost your DPS midgame. If you ask me i'd recommend ya any item of damage+hp.

GinWinsky on BR Servers of League of Legends <3