can't recall much about tzeentch... but i can pick my manager's brain a bit tomorrow!
the soulgrinder is basically a more assaulty defiler without a battle cannon IIRC.
and the daemons are bloody pricey as it seems they're going to be a mainly even split between plastic & metal models. (most other armies are roughly a 75:25 split between plastic and metal)
of corse, if you want to feel a bit better about it, just remember you're not building a dark elf army which has 1 entire plastic set in total! (so you're not building the all metal army at least!
)
cheers!