I think there're still a lot of unknowns. You can read the actual language of the proposed bill here
GREEN Act of 2021 (H.R. 848) , but my understanding is this is just a proposed version, which hasn't been negotiated or voted on yet. And it likely won't look exactly like this after it's chewed up and spat out the DC machine months later. The section on electric vehicles is Title IV, section 401.
To answer your questions, I haven't see anything in there saying that it would be retroactively applied to purchases before the bill passed. But who knows... here's the section about applicability of dates "In the case of any new qualified plug-in electric drive motor vehicle
sold after the date of the enactment of the GREEN Act of 2021—"
No income limit or car $ value cap from what I've seen (assuming you at least have a tax liability of $7k to take advantage of the credit).
I saw some rumormill type of article recently
https://www.washingtonpost.com/us-policy/2021/03/22/infrastructure-biden-drug-reform/ stating that Biden will be pushing an infrastructure bill with some climate initiatives in it, so maybe this bill and his infrastructure bill will be combined... but really your guess is as good as anyone's unless they are directly speaking to the lawmakers.