I think 12k sounds like a fair price for what info you've provided. I gather the picture is of the motor that's now in it? I would take a serious look (undercarriage, frame rails, firewall ect) for more rust issues. The rust around the windows may end up with you needing a new roof skin. There were other areas showing rust which may be minor or an indication of a more serious problem. I paid 12k for my '74 Challenger, and it was much like yours with a transplanted 440, but I bought a one-owner garage kept car. On the surface, it looks like a very nice driver quality car and it may in fact be a very reasonable purchase, just be sure to check out the rest of the car for more rust and other mechanical issues & check out the drivetrain for good operability. Good luck & keep us posted!
Matt B.