I have Vol I (A-7E/D/H)and Vol III (A-7K/TA-7's/EA-7L). But, I've got them one at a time when they were released, and because of personal tastes regarding the versions included.
Vol II is dealing with the initial A-7's before they were fitted with the HUD/modern NAV-ATTACK systems and also they are powered by P&W TF-30's (weaker than the TF-41).
Vol III includes some additional features (i.e brake chute), rear seat camera and of course the rear seat.
ANY OF THEM IS SUPERB, however I'd suggest that, if you don't want to buy all of them, to take either a single seat volume (VolI or Vol II) AND the two seat Vol III, or, if you don't want to spend $70 buy Vol III.
Regardless of your choice, I think you'll be proud of it, since RAZBAM's A-7's are iconic A-7 releases and their support is not just commercialy good, but personal and friendly.