That's basically what I use. Except that I always get SoulStealer and decide between Rod of Ages and Zhonya's depending on how good I do.
First items: The AP/HP ring (Baldor?) and a HP potion. I ussualy blue pill back around level 5/6 to buy me some boots.
1) Sorcery Boots
3) Rod if I'm doing well (So don't need the AP spike) for survivability and staying power.
3) Zhonya's if my SS is remaining rather empty; I need that AP to remain a threat.
4) Whichever I didn't get at 3.
5) Ussualy I have a Tear now, might upgrade it to Archangel staff right away. Provides both Mana and AP. I lile.
6) Lich bane. Might get this before Archangel for no real reason.
I level Pulse whenever I can, and harras/last hit with Q. Do not try to stack riftwalk for the damage. It's not worth it early on. Use it just for mobility. My masteries are 9/0/21 grabbing magic pent in offence and mana regeneration, movement speed and reduced cool down in utility.
Runes are Magic Pent Reds, Dodge Yellows, Cooldown Blues, Health Quints. Not really optimised for Kass, its what I use for all magic damage dealers.
Summoner spells are Ignite and Clarity.
Ignite gives that extra push to bring them down; You need some kills for SS and with the recent trend at Riot to make heroes with build-in HP regen its often a valid choice.
Clarity gives me mana on command and has low CD. Why waste precious ability power gold on mana, if there's a spell that gives me more. :O!
Mind you, I only play unranked games.