This will -by itself- be an eight mana 0/0 or anything up to a three mana 5/5, but as a 6-for-1... never costing less than three mana.

Nope. Unplayable in any of its modes. One mana and a +1/+1 counter for a card might be a good deal, but the fact that you have to invest a lot of cards or a lot of mana to get a 0/0 first is a dealbreaker.

This would be kinda cool if it allowed you to discard a single card (in that case preferably chosen at random) and defines X as that cards converted mana cost. The wording would be tricky, but managable and at least you might hit a high cmc and get a sizable three mana 2-for-1.

It is also noteworthy that Hydras are a green iconic creature type and seems ill-fitting here, though Hydras used to be mainly red and an Eternal Hydra or something like that would be neat.

Investing cards into a Hydra could be much cooler to fit the colors e. g. > "~ costs less to cast for each card in your hand.
As an additional cost to cast ~, discard X cards.
~ enters the battlefield with a +1/+1 counter on it for each card less than seven you have in your hand.
When ~ enters the battlefield, draw X cards."

IIRC the rules were adjusted to make this wording work at some point.

Now the final cost of that creature probably would still have to be less than eight to be attractive, but it gives you much more to work with than "Hopefully you had a terrible day of half a dozen dead draws, so you can get a mildly undrecost creature out of it and are ready to ignore that Zombie Infestation is a better use of your cards.