You must not be good at google. http://lmgtfy.com/?q=n42+head+and+block+datsun
N42 can be a head and block from the L series engine line. Most of the links in the lmgtfy will clarify that.
As for the damage, I'd be more concerned with the straightness of the frame rails on that side. Your concerns about a crappy POR 15 job over rust is also very valid. You'd have to get underneath and check frame rails.
From the look of the add, it seems it have at least the basics (seats, center console, and dash). What are your intended uses for the car?