This pretty much says it.

This build simply does not work, you are spending +7k gold before getting any ability power! Yes sure you won't die, because no decent player will focus you. You will be worthless in teamfights. You should defitely swap the order of purchase, damage first then tankiness.

The build seems rather solid, I think it would work out well, but personally it´s just not my playstyle with Kassadin :)
I´m happy enough to have a flash spell, saving me in 60% of all times. The dmg is after my opinion too low in the early game.
Thinkig about the average money you can earn in a game, lying somewhere around 10k (well farmed, some kills, whatssoever) you just got an Archangels Staff to do some Damage.
For me this is just way to low, your whole build rocks I think, but it rocks too late, and when I have a Kassadin in my team I want him to deal decent dmg ;)
So when I would play this build, I rather would get the Archangels Staff before the Guardian Angel, then a Blasting Wand and then the Guardian Angel.
Just some feedback for this :)

If I get some positive feedback I'll go back, clean this up, and add some photos. Any constructive criticism would be appreciated as well.