Well, the thing is, guns are consistent, effective, and able to be used by anyone. Magic, even (or perhaps especially) with the favour of Tzeentch, is random, and it is generally limited to psykers. A sniper rifle will always put a hole in the head you were aiming at, whereas, say, a head-exploding spell might make that head go boom, it might even make the heads of that enemy's whole squad go boom. Or it might make your psyker melt, or burst the heads of some of your cultists, or summon a hostile Bloodletter into the middle of your army.
I'm not saying that we shouldn't look into making daemon-golems, I'm suggesting we focus on getting some basic, practical equipment for to improve our basic grunts (and possibly Chaos Space Marines) first. I was thinking along the lines of the Goliath mine trucks (Which is a good idea by the way and has my support). There's also the tech development costs to consider - stuff we can nick from the PDF, mines, or wherever is gonna take much fewer dice than inventing any kind of warp trickery.
Finally, mundane weaponry, even bolters and grenades, also give us a much higher chance of passing off a firefight as (possibly very well equipped) gangers fighting, while that sort of blatant warpcraft will raise a lot of suspicions very quickly.
EDIT: We haven't decided on a cult name yet, have we? Only suggestion I recall was my Cult of the Crimson Blade, which isn't really appropriate for the god that was chosen. The Cult of the Burning Eye perhaps?