Okay, you are good to go with your understructure. Just checking. Hate to see this project go forward and have discovered later that the floor was too weak. The 3/4" ply is ok for using 1/2" cement board. Some may say you could use 1/4" cement board, but I don't like skirting the minimums for a floor that should last generations.
You can use unmodifed under lots of the cement boards in many occasions. There are
times when the floor thickness is running at a minimum, or the joists are spaced out further that using modified is required. There is nothing wrong with using modified in this case, it just cost a few bucks more. If you want to use modified under there......go ahead.....but I think I'd use Versabond to still save you some dough.
Dan, because open bags of thinset start collecting moisture right after they are opened, they don't last long (30 days opened). If I use 2 types of mortar on a job, the chances of having open bags when I'm done is twice as great as if I use one type of mortar. I do complete kitchen and bath remodeling so there is always weeks between tiling projects. For this reason, I use a modified for everything....less open bags that want to go bad on me and less mess in the truck.
Hope that sheds more light on the subject.