Besides parts and ease of restoration, another thing to think about is the size of the tractor.  We have lots of big tractors that are my favorites, but I wouldnt recommend someone have them with out the right equipment (meaning truck/trailer, shop, tools).  Also, with a smaller tractor, parts are lighter and easier to handle by yourself.  What I mean is that with a 70, you could take a good jack, some good stands, or boards, and take of a rear wheel and replace the tire by your self, no problem.  Try doing that with something like our 995 that has 1500lb. cast rear wheel centers AND fluid in the tires.  You need a shop with a hoist, and a semi to haul it to shows on.
Therefore, in my long and drawn out fashion, I agree that you need a model like a 60 or 70, or 66 0r 77.  I think that I would recommend a 66 RowCrop Gas.
Small, simple, classic.