EDIT: After testing, I've discovered the answers to both questions are "BASE STATS ONLY!" No using fox's cunning to qualify you for Combat Expertise, for example, nor using an Amulet of Wisdom to let yourself cast higher-level spells due to stat reqs. In short, start with the base stats you plan to have throughout the game! _ _ _ _ _ _ _ _ _ For purposes of meeting prerequisites for feats, spell levels, etc., do I need a base ability score of that much, or a total score? For example, if I have 12 base STR but am under bull's strength, can I take Power Attack at level up? If I'm a level 5 Wizard with 11 INT but have a Headband of INT +2, am I able to cast level 3 spells?
I haven't tested this in-game, but enhancement bonuses should not allow for feat selection, but should allow for spell selection. This is based on my experience with the PnP.
But having an Ioun Stone, Periapt of Wisdom, or the like will allow your character to gain the bonus spells per level as a spellcaster. But if you remove the item, the spells will go away and you will have to rest again to relearn them. In your example, the Wiz with 11 INT won't be able to cast anything but level 1 spells, but if he had a 16, and then put on the Headband for an 18, he would gain an extra bonus level 1 & 2 spell per day.
