One hand (dual daggers), light armor, sneak.

I was also thinking of using illusion and conjuration. How much magicka would I need to do this?

EDIT: And I am trying to decide between alchemy or enchant.

I'd take alchemy for the poisons and invisibility potions.

I never cast spells with my assassin so I can't advise on magicka, sorry.

I RP a "pure" (as they say) assassin archetype, sans magic/enchanting and such.

Good luck though.

oh my do it all new Assassin!! why choose between the all the perks?

If your focusing on sneak attacks dual wielding daggers is redundant. I'd expand alchemy for the variety of effects.

Don't worry about build. Want to be an assassin? Join the Dark Brotherhood. Done!

Everybody does realize that "assassin" is a job and not a build, yes? I mean, you can be an assassin using a guy who has any mix of M/H/S and wields any type of weapon and uses any type of magic and wears any type of armor.

In the interest of not appearing to be a jack-ass (which I'm sure it looks like right now), exactly what kind of play style are you going for? Stealth? Why would you want Conjuration? What purpose would that fill in your assassin job? Are you a "hands-off" assassin who wants creatures to do your job for you? If not, Conjuration is going to work at cross-purposes to the "stealth" concept.

And if you're going to stay stealthy and quiet, you probably want to invest in Alchemy, not Enchanting, so that you can make very strong poisons and immobilize your enemies if your first strike doesn't kill. As an assassin, nothing beats the Paralyze/Ravage Health poison combination.

I'm playing a character very similar to what you're doing. I went with alchemy rather than enchanting. My goal is to slowly strip myself of all the enchanted gear I've been relying on, replacing it with potions.

I started my build thinking I was going to go into illusion magic as well, but I haven't really relied on it much. Poisons also will accomplish what many illusion spells will accomplish--frenzy, calm, etc. (Although having to deliver the poison with a weapon can be a drawback. My archery is 100, so it's tough to deliver a poison via arrow without killing the target.)

My assassin has about 200 magicka I believe. Invisibilty costs 150~ (I think) with the expert perk. I only use Illusion on that guy, so I can't comment on conjuration.

I went enchanting and made some nice sneak/muffle/archery/regen type stuff. I think one handed enchants are pointless due to the sneak modifier being so high once you get those perks.

In terms of alchemy I don't know. You can poison people which is nice, but in terms of actual combat I don't see the need. If you use a bow at all you can do some very intense damage from afar and once you get up close you are going to be 1-hitting stuff with backstabs.

Alchemy all the way. I personally wouldn't bother with any magical perks since I prefer to play my assassins without casting spells. I would recommend the pickpocket skill though. One the best things to do when playing an assassin is to place poisons on NPC's and watch them die. A true assassin leaves no trace that they were ever there.