If you ever want magic resist on Rammus, go for an Abyssal Scepter. 57 Magic Resist, -20 Magic Resist for enemies, and 70 AP really helps your Powerball and Tremors.
What I generally go for now is 2x Heart of Gold by the 12-15 minute mark, then keep them for the majority of the game. Your farming ability isn't really great, thus during roaming phase the gold generation really helps. You can sell one later on if needed, and the other can be straight upgraded to Randuin's. Basically after 10-15 minutes holding HoG, you're gaining profit. The extra survivability helps you stay in field as well.
As for starting items, Cloth Armor for +18 armor giving you a extra 5 damage from your passive, 3 health pots, 1 mana pot is generally a good start.