With regard to the AE-II vs. AE-III, the latter uses an LCD screen and thus has significantly lower power consumption and increased battery life.
Interesting note about the respective prices. In Tokyo the Pentax gear is generally cheaper or about the same. Old beat up 6x7 bodies are under $50 and with a little love and care they generally work well. Lenses are almost all under $100 if you are willing to settle for the Tak series. The 645 bodies are about $100 for the original, $300 for the N and a lot more for the Nii. Lenses are very reasonable too if you are ok with the A series. A used ETRS or ETRSi in decent condition with a standard lens will also run about $400. Fujiya has two of them listed today!