$4 Action-Attack
+$2
Each other player gains a curse and takes 2 coin tokens.
Nice idea, but this seems way weaker than Sea Hag. In general, giving everyone (including yourself) the same bonus is better for them than for you (especially true in a terminal action), unless the bonus has specific synergy with the rest of the card or with a particular deck (for instance, giving everyone Copper benefits sloggers but hurts engineers). Basically, why Council Room is worse than Smitty if you don't have discard attacks, unless you really need the Buy.
In this case, the bonus for the rest is +$2 without spending the action AND it can be saved for later if it happens to be timed wrong for them. Timing of the bonus is the only thing in favor of the player playing the card, but in this case, that's closer to a disadvantage.
Also, Sea Hag puts the Curse on top of the opponent's deck, so the attack on this card is weaker.
I would give them just 1 coin.
For the curse on-buy, I'd said this would be done just if overpaid (just to mantain the theme), but I bet we won't see another curse on-buy (especially since its been mentioned that the benefit of overpaying was going to be proportional to the overpaid amount, and giving out more than 1 curse on-buy seems terrible). I guess it could be something like "for each $3 you overpay, curse the others", so that multiple curses are really difficult to do, even impossible early in the game, and difficult later because you are going to be single-cursed a lot at the beginning.