The movement speed from the boots will help you to juke skill shot abilites from other AP carries and the health pots will help you to stay in lane.
I went mostly for flat ap runes and some magic resistance so I don't have to buy a amplifying tomb. the main reason to not get a dorans ring is that you won't get a lot of sustain because other ap casters will have an easy time to harass you and deny your farming. Use your null sphere to harass your lane opponent and just auto attack to get last hits. You can last hit with null sphere on the bigger minions though.

You should not engage the team fights unless your tank is their. Kassadin is all about bursting down the ap casters and the ad carries. The combo order is Q to silence them and then use your E to catch up and then R to get close enough and deal finish them off, use your W directly after to take down the last of their hp if they are still alive.

Kassadin doesn't need cooldown reduction because all of his abilites works as a bursting combo and he shouldn't spam his abilites all the time.

A good tip to use your ultimate is to teleport behind them. They won't except it and you will get some free damage. Use it to juke abilites as well.

If you don't need flash I would suggest switching it to cleanse