The way I have the items set up is grab either a Sapphire Crystal and two Health Potions or a Meki Pendant and two Health Potions. I prefer the Meki Pendant because just like the seals it's great to have that early game mana regeneration because he is REALLY mana hungry. Once your get that Tear of the Goddess you will be mint though. If the team has a lot of stuns and slows I would suggest the Mercury's Treads but if you want more damage go for the Sorcerer's Shoes. Get the Rod of Ages around 26 minutes that's perfect because most games now end around 40 minutes in so you'll have the ten minutes needed for it to be complete in no times. You could get the Mejai's first if you're confident in your Kassadin skills or you know you're going to get a bunch of kills and assists. Archangel's Staff is just great for the final item, more mana, more damage and it's passive is awesome. If you have all these items and still want another item I would suggest either the Abyssal Sceptre or Void Staff.