Why do you need Alteration? None of the spells in that school are very useful for an Assassin, except for Detect Dead/Life (but there's a shout for that) and (maybe) Water Breathing.
Race? High Elf for bonus to Magicka, Khajiit for bonus to Sneak, any race will work, really. I myself use an Orc for an assassin, as their racial ability in combination with the Assassin's blade perk and DB gloves allow me to do 60 times back stab damage, but that makes things really easy, so.. your choice, man. Choose the race that looks the best!
I'd focus on Sneak, of course, Illusion, One-Handed, and maybe Alchemy and Enchanting. When I play an assassin, I don't put any points into health and don't put any points in any type of armor. This makes remaining undetected crucial to surviving. I'd also keep a bow with you at all times to lead enemies away from choke points.
That's abut it. Good luck, bro!
