All good questions!
I don't find that I need to alter monster stats when using the OSE Monster Book. The same goes for the spells (though obviously, you need to adapt them to the [dice] [sum] system which is very easy once you do it a few times).
You do need to alter treasure. I don't bother on a monster-to-monster basis, but when stocking a dungeon.
Wizards: I've never thought about it actually haha. Whatever you like best I guess? I've always assumed at least 1 free hand and the ability to speak. Thus being tied up or under the effects of a Silence spell they cannot speak
As long as a wizard has the Item Slots to carry heavy armor they can wear it.