I entertained an identical question about a week ago...
decided to go with ghouls (should get here in a few days)...I plan to use the legs/body (not the head) in most cases with between 1 and 3 arms coming out from different angles
depending how it looks when I get them in front of me I am a little hopeful that I may even be able to turn the box of 10 into more (maybe 13-14 horrors, with some being legs and arms basically, others being torsos and arms without legs...it may not look convincing though, haven't tried it yet)
on a similar topic, i have a plan to turn ghouls into furies (again, don't want to spend $16.50 or whatever it is for 2 metal models...in this case i am ripping off the idea from this thread:
http://warhammer.org.uk/phpBB/viewtopic.php?f=4&t=63093 - I like his ghoul furies)
i don't know if u plan to include flesh hounds in your army, but based on your feelings about horrors (I assume you are well aware of this) you will almost certainly want to buy the Chaos War Hounds from WoC, giving you 10 plastic models for $20ish, as opposed to 5 metal models for $40ish
I also wanted cheaper/plastic fiends, and will probably try to copy Hexantanz's spidernettes (tho almost certainly less successfully)
also have been turning over a thought for Seekers of slaanesh to be another daemonette centaur using the elven steeds as lower half...and probably adding spikes or tentacles to the steeds, maybe replacing the tails with a snake/whip/stinger/claw kinda thing
another idea I got from this forum (I think also from Hexantanz) is using the Chaos Spawn kit to make Daemon Princes - i may look into that especially cuz it will then give me tons of chaos spikes and tentacle bits to put on other stuff as well (like my theoretical seekers)
anyway, just sharing my thoughts as I imagine we are about to undertake similar projects...gl