where
The key questions are answered in the OP of the greenlight announcement thread
Q: Will I have to buy the game again?
A: No. Your current Blockland key will work on steam (assuming this is possible, which it should be).
Q: Will I get to keep my BLID if Blockland gets on steam?
A: Yes
As for overlay questions, the first three have been answered by the many people who already run blockland through steam
The last one has been addressed by Ephi a few times