I feel like sometimes this is going to be very overpowered (e.g. in the case of Worker's Village / Grand Market, as mentioned above), and other times it's going to be completely useless (opponent always only has 1 buy, or is always using all their buys). This might be an oversimplification, but it's how it feels to me. Even other $5 attacks that do "nothing" usually have additional benefit, whether it's draw, virtual economy, whatever (exception: Saboteur). So perhaps giving more vanilla benefits than what you already have (e.g. now and at the start of your next turn, +1 card) would be necessary. Or maybe limit it to max 1 curse, and make it $4. Or maybe go by extra actions instead of extra buys. Not sure.
EDIT:
I thought of something else.. I sort of see having extra actions at the end of your turn, and having extra buys, and having drawn your deck in a single turn, etc. (obviously I'm talking about an engine here) are often consequences of playing well. That is, it may be difficult to set something like this up, and is often associated with skilled Dominion players and expert-level Dominion play. Therefore, this attack is, in a way, punishing good play. And I don't like that. I think good play should be rewarded, not punished.