My main abilities:

1-handed
sneak
alchemy
illusion

Of course there will be more than that, but that's the core. I'm really resisting taking archery.
So far I'm only level 10, but to this point I find myself rarely ever dual wielding. Will I need dual flurry later on?

You can't cast a spell and attack at the same time, just something to consider.

Take a look at http://www.gamesas.com/index.php?showtopic=1290251. We discussed the pros and cons of Illusion on an assassin character in a bit of detail in there.

As for dual wield, I use it on my assassin'ish character, though mostly because it gives me another playstyle in addition to backstab and archery (basically three builds rolled into one). The damage gets really insane once you get all the dual-wield perks, and with some life leech (absorb health) on the weapons, it's quite powerful (and fast paced).