I think taking Debt as the downside of a card is fine. With Scientist, i was originally worried you'd maybe trash all your treasures and then have debt and no way of paying it back, but by now i think that's unrealistic. Not even Possession can do that to you, now.
Whether Debt is better or worse than discarding a card depends on the phase of the game. At the beginning, it's worse, as your coin density is below $1. In the middle of the game, it's better. In the endgame, it goes down again. Generally, Debt is worse the worse your deck is, while sifting may help in that situation. I tend to say you are generally better off taking debt.
Here's my other Debt-taking card, by the way:
Investor, $4, Action
Take 3 Debt.
Take 3 coin tokens.
Your Investment goes in a similar direction as my Scientist. Scientist should probably cost more, and Investment might need to do that, too. It compares well to Embassy once your deck got good.
Generally, taking Debt on choice seems okay. I like taking debt as a way to defend yourself, but of course that's not much better than getting hit by Cutpurse normally.
Not sure what i think about Pawnshop, but it's probably fine. Might also be a little too strong. I mean, Estate to Gold for $4, and Gold to Province for $2, that's rather fast. Cool idea, though, maybe i totally misjudge its power.