Another cool and interesting card. Again, most of the things might have already been said.
I think it can best compared to Chapel. It's a great trasher in the first couple of turns, but gets really weak really fast.
Buying and Overpaying it on a 5/2 is really great as you may hit 2 Estates and can get a Doctor/$4 opening with 2 Estates already out of your deck. You might draw it even on turn 2 and trash 3 Coppers, so you have already trashed half of your starting cards, this is way faster deck thinning than with Chapel.
Other than that, it doesn't seem so strong. Trashing cards that aren't in your hand is great, no doubt about that because you can trash and still have probably enough money to buy a Silver or better. Naming a card at least has no drawback hitting 3 Colonies (at least that's how I understand it). But you really have to know what to name what makes it hard to guess in the mid-game. You're probably most naming Coppers, or Curse in a cursing game. If you couldn't find one, you at least don't have a Loan-like disadvantage in discarding good cards and just have a little bit of deck inspection, but all for the cost of an action.
The overpaying is great at the beginning, but I'm not sure when I want to pass a Gold or even a Province just to trash a few cards. If you have a lot of money in hand that means that you have a lot of bad cards left in your deck, that's true, but I rather buy a Warehouse (granted that it's in the kingdom) to cycle through my bad cards than getting a mostly dead card in my deck to just trash a little bit. I mean on-buy trashing is not only for the cost of money, also for the cost of getting one card in your deck, that isn't that useful. This means you have to trash at least 2 cards to make it worth it which means that you have to pay at least $5. Not being able to look at all the cards that you overpay for makes it even less powerful if you pay $5 and draw a Gold first. I don't think that the overpaying is that useful after turn 5, but having the possibility is great and something you have to at least consider.