Thom, I would have bought it too. And at this stage, I would continue to move forward. To resurrect that motor, an enormous commitment will be required of you. The patience, time and money required will be substantial in comparison to your average OMC motor from the same era. That said, when it’s all over, you’ll have what I consider the "holy grail" of 4 cylinder mercurys (I’m still on my quest to find one).
I would consult the guys at JOMS in addition to asking your questions on this board. I have found many of the knowledgeable Mercury specialists and experts there do not frequent this site.
Good luck and keep us posted!
Scott