Here's my reasoning behind my card.
My first decision was to pick the price. I decided to not due $6 due to Harem/Nobles/Farmland/Fairground. I really didn't feel like designing a more expensive card and just decided that I enjoy games where there are VP cards at costs other than standard VP cards. This left $3 and $4 for me to pick between. The mechanic came to me in some of my numerous games where I've drawn KC/TR dead numerous times. I thought, what if there was a card, that allowed me to get that KC back on my deck without needing to have purchased such card earlier. Naturally, a buy/gain mechanic allowing top decking was required. I decided to include both action and treasure to make the effect occur regardless of whether an action, treasure or both had been played. Midgame, you can buy it to align action cards with careful deck control. There is also the option to buy it multiple times with attack cards in play such as Goons/Witch in order to play them as much as possible. And finally, end game, they provide a unique contrast to Duchy. With $5+, you need to decide whether the extra VP from Duchy is more important or if the ability to put one of your good cards on top is more important.
EDIT: Forgot to say why $4! I kinda touch on it, but I wanted the card to be more of an alternative to Duchy rather than an alternative to Estate