As Matt said - Rust! Look in the rocker panel area, pull the carpet back in the floor boards, look in the rear trunk area. Look to make sure it has a fuel door, look at the conditions of the bumpers and the fenders (often the fenders will be pushed in where the bumper corners contact them). Radio and/or antenna are a plus! Condition of all the rubber seals (windows, doors, etc) - most of them are pretty bad but if it's good, it's a keeper! Hub caps are good to have. See if it has a diaphragm carb or the piston one - the piston one is better.
Thanks for the slightly off topic advice on buying a Honda N600. I now have a bright yellow (most recent of probably numerous repaints) 1971 N600 that makes my wife's Metro look huge as they sit side by side.
Seems to run well, solid floors, hub caps are there, shop and owner's manuals, a little minor surface rust and a few dings and dents, and lots of vinyl stickers all over as it was used to advertise a local business. Should be drivable the way it sits for the summer. Needs a pair of tires, some window rubber, has a headlight electrical problem, but nothing that seems major.
Haven't seen much activity on this thread lately...did you ever get the rebuilt engine in the car, Matt?
I'm looking into getting a 1971 N600, one of my customers has 3 of them and is looking to (possibly) sell one soon. They're all in reasonably good shape for being almost 40 years old, although one is substantially better than the other two. That's the one I want...will keep everyone posted.