Tanking with Nami is a VERY bad idea. She's just not built for it, and your item slots and gold can be put towards a much better investment.
Aside from the Spellthief items, she should specialize in building heavy AP, mana, mana regen, cooldown reduction and movement speed. Everything she has scales off of AP, including her passive, and all of her spells, if they hit an ally, boost their speed, even her stun and ult. She's like Sona in the respect that she dishes out damage in small, powerful yet unsustainable bursts. Only get in close to the enemy if you're helping your ally or planning to stun or poke them, and stay around for a small bit longer if they're actually stunned so you can lay down an extra hit or two.
Her E is technically a damage boost, so building AP will increase your carry's damage. Building a Lich Bane will capitalize on your bursting power, and the Censer will increase her movement speed and the DPS of her allies. The Crucible is almost a must-have on her, and while any mana item will work, ones that increase her utility and spells are the best options. Tanking is an absolute no no, and if she's the tankiest on the team, then there's an issue with the team itself. Any standard mage/support shoes (Sorcerer's, Mobility, Lucidity and Swiftness) are all fine decisions on her, depending on what's expected of you and what the rest of your build is like (Heavy AP + Lich Bane: Sorcerer's; Low cooldowns: Lucidity; High mobility: Mobility or Swiftness)