I hate to say it, but I think that 4th ed might be a tighter fit. I think the at will/ conflict/ per day thing might work well to model some of the conflicts in the books. Harry tends to rely on a lot of the same meat and potatoes spells. When shit hits the fan, he busts out a smaller variety of more powerful effects. When shit really, well, and truly hits the fan, he busts out some of the really powerful effects. It also has the advantage of giving everyone a couple of cool things each even at starting level.
This is why I love using Mutants and Masterminds. Basically Harry has a few spells (you'll see soon, he's actually been a bear and a half to stat up correctly) in a defined magic array. Those rare things he uses? Those are power stunts he uses from all those hero points he gets from the countless complications he has. The really powerful effects? Every notice how he gets really tired after that? He slaps the Tiring flaw on the spell to get more points out of it and really pull out a powerful bash.
And just to whet some appetites, here's the Wizard Template:
Wizard Traits (Container, Passive 3) (Power Feats: Innate)
Soul Gaze (Super-Senses 8) (Postcognition, Precognition; Flaws: Limited (Subject You Gaze), Range (Touch), Side-Effect 2 (Always - Subject Gets to Soul Gaze You); Power Feats: Perfect Recall; Descriptors: Magic, Psychic)
Enhanced Recovery (Regeneration 4) (Recovery Bonus 4 (+4 to Recover); Power Feats: Regrowth, Persistent; Descriptors: Biological, Magic)
Wizard Sight (Super-Senses 14) (Precognition (Flaw: Uncontrolled), True Sight; Flaws: Check Required (Concentration), Side-Effect (Sometimes - Psychic Scarring); Power Feats: Perfect Recall; Descriptors: Magic, Mind)
Boosted Immune System (Immunity 2) (Aging, Disease; Flaws: Limited - Half Effect)
New Power FeatPerfect Recall: You can remember everything you see or experience with this power as if you had the Eidetic Memory feat, but without the need to actually make a roll to recall the information. Anything you see or learn with a power using this feat stays with you forever, for good or ill.
Just a few notes on this. It was actually harder to mimic the basic attributes of fully-fledged wizards than I thought, but in the end I think this works really well. The Soul Gaze was interesting, as I almost was going to use Mind Reading, but I think that post and precognition work a lot better. Those powers are rather symbolic anyway, so it was a nice fit. The touch range is simply to denote that you have to be very close to your subject to use it. The enhanced recover is interesting, as wizard's don't really heal any faster than anyone else, they just heal more reliability (the recovery bonus) and they can recover from things most people can't (Regrowth and Persistent). Due to a quirk of the M&M rules, they probably heal faster than Harry actually does, but it's a pretty close match.
The Boosted Immune system mimics the slow aging of wizards and the fact that I've never once actually seen one of them sick (that I can remember). Well, Harry might have been sick once or twice, but that was overwork I think. But that's why it's limited to half effect. Actually, the Sight was the easiest thing to mimic for me. They just get True Sight and an uncontrolled Precognition once the sight begins to "come in." I would model the Psychic Scarring as a Mental Stun effect at a level equal to the Power Level of the nasty thing you saw. So minor horrors you can save against pretty easily, but things like Shagnasty are going to throw you for a loop.
