For the second part, I want the caster to only gain X life, I don't want them to gain double X life. As the card is worded, does the caster gain any life at all if it targets a green creature? Need some help with wording this. Thanks.

­Drain Life++ As for the wording to get what you want; something like: ~ deals X damage to target creature, and you gain life. If that creature is green, ~ deals an additional X damage to it.

But I think "Find something simpler to do instead" is probably the right approach here. Does the card really need to be that much better than standard black life draining?

This is just cleaner and this card was originally envisioned as a green hoser anyways.

I have a new question, though. Is the casting cost fair or is this card now OP?

Looks fine to me. It's Swallowing Plague with a benefit versus green creatures, and I don't remember Swallowing Plague devastating tournaments (though it's a fine card).

You can also put a Saproling on the table, use this, and 'sacrifice' the saproling for double Stream of Life. Cute, but not broken.