Sorry about my English.
I just played Nasus and bought Ravenous Hydra once I got around 200 armor and 3k hp with 200 something bonus damage on my siphoning strike. The item returned around 700 hp when activated on minions with siphoning strike. Just wondering if the item is worth buying? The life steal and damage's awesome, but the item itself's pretty expensive. Should I get it or ignore it and stack hp and armor instead ? Thx !!

You should never buy a damage item on Nasus that isn't Trinity Force.

the aoe effect does not take your SS into account, so your still doing small amounts of damage to your opponent in the aoe aspect. your better off getting CDR items like black cleaver than using this item.

The new Hydra is not as good as the old tiamat for Nasus. Old tiamat used to splash for part of the attacks damage letting tiamat benefit from his Q, but the new tiamat and hydra use base AD. Hydra, however, isn't a terrible item on Nasus. It works well with his passive and his ult. However, it would be a late game item, and should not be an early but since it can make it difficult to farm his Q.