For hunting down Warmachines, we don't have a ton of options. Being denied harpies hurts a bit.
However, Marauder horsemen as light cav./ vanguard as CORE is amazing. Give them some flails and throwing spears and you have a very good harrying unit that costs very little.
Warhounds are decent, but not that effective, even if they survive to get into combat. (which, they most often do not). They are very cheap and good as disposable units: setting up charges, taking some volleys, pulling fanatics etc.
Spawns are too pricey for their cost and cannot be relied on. On bad Movement roll, and you have a useless unit. Seeing that most units will outrank it, you lose out on much of it's potency.
Chariots are ok. Seeing as we're WoC and have VERY nice Initiative, we really don't need chariots. Their IMPACT hits is what make them useful. After their impact hits, they have very little staying power. Our Warriors are more potent and cheaper than the chariot (which is about 100is points...easily 5-8 warriors), they will work much better than the cumbersome chariot.
I use chariots w/ my beastmen as our I is low and they are CORE for us and 20+% cheaper than yours. So beastmen can field large amounts or many as flanking units.
Honestly, I'd try out some horsemen. They're cheap and if they die, you lost 80-100ish points. Couple them with some cheap hounds and you can cause some nice panic. Just don't take away from your main lines.
Double Hellcannons are a sight to see and can smash the living bejezus out of unsuspecting armies.