My plan has been to convert some, but you're probably better off press-moulding some of the critical models given how much the original daemons cost.
My plans are:
Slaanesh chariot: Dark Elf chariot (from character pack) with GW daemonettes and seeker steeds
Tzeentch chariot: Flamers (from the Horror command strips) with Screamers and some greenstuff work done to old disk-shields to make the chariots. These are probably the easiest to make as the flamers could be scratch built relatively easily.
Khorne chariot: Chaos chariot, Juggernaught from the character pack and Bloodletters.
Nurgle chariot: Orc chariot (from the character pack), with Beasts of Nurgle and Plaguebearers
One other version I've seen used Chaos Chariots with Chaos Hounds and Bloodletters- that may be one of the cheapest and easiest to make.