This is my current build:
http://www.pathofexile.com/passive-...zwuzDbdVP14bb59xX3nffsOL35BHmgeq67Bjwa_ll-eg=
Currently using a 4-link Flameblast, Rarity (yaaaay), Faster Cast, Life Leech. In "serious mode", the next ones would be: Concentrated Effect, Increased Area of effect (both of these overpower the slight gain from Faster Casting by a LARGE margin, they turn for example a 900 dps stage 1 Flameblast into an 1440 dps one...), and finally Fire Penetration if I get an 5l- or more.
I want to take the shield passives (Ratpith Globe atm), and will move out some cast speed points once I find enough resists on my gear to equip the +13% cast speed, +50% spell block unique. Even now, one cast is 0.15 sec (without having any cast speed on my dagger, another non-optimal thing - but hey, it has 77% spell damage and tons of crit).
I am not going the ignite route, although with the right dagger, it could work. I also do not spec into crits, predictable damage is good enough as of now.
With your planned playstyle, if the enemies are affected by elemental equilibrum (due to a freezing pulse or arctic breath or ice spear totem), you will not need more than stage 2-3 casts to kill non-elites, and not more than 6-7 to seriously damage elites as well. So that can work.