in a pure DH army, you'll want a couple of squads of stormies for numbers... they're dirt cheap and still have a solid BS. don't bother with a vet sergeant upgrade though as if your guardsmen are in combat then something is very, very wrong! (oh, and paying marine prices for basic humans is crap too!
)
mobility is also going to be a problem in a pure 'hunter force as grey knights themselves don't have transports... (land raiders don't count much due to their cost and they're non-dedicated.)
cheers!