I dont want to write so much, but if you need more magic resist ore amor you also can take items like Force of Nature ore Thornmail. You can try to take Glacial Shroud instead of Manamune, in late game you can build Frozen Heart. My Guide is a DPS Offtank, but if you have Trnity Force your DMG is god, after Trinity you can play with Full Tank items. If you dont want to buy Warmogs, buy an item with live,too. You need the live in the late game!
If you look at the Damage on the top of this page you can see there is only 169, but thats not right, they forgot the 62 DMG of Atma´s Impaler! In Mmy second build you play much more Tank. you can buy Force of Nature before Randuin´s omen if you need more magic resist. Philosopher´s Stone can you sale if you have Glacial shroud and Trinity, Philosopher´s Stone is only for the early laning. You have Heal Regen and Mana Regen with this item, thats god in eary laning.