Very interesting. I am intrigued!
OK, here is my take on this.
If I were going to play in a one-shot type of game, I would take your guy hands down. Light armor isn't a big deal for what this guy could do, unless he can get some magical armor. However, being able to use a shield is a nice bonus! Carry it until you have used up your Arcane offense spells. The improved HP and flexibility of spells is far too appealing! We are talking about the same average HP as a 6th level Cleric. He does have a lower BAB, but no worse than a 6th level Wizard. The lower skills isn't that big a deal.
If I were going to play this guy in a campaign, it might be a little harder decision. It looks like there are some nice entry level costs, but I am guessing that at "12th level" it isn't just as simple as assuming he will have the spell casting ability of a 10th level wizard and a 10th level cleric. That might make the concept viable (and even quite strong) at 6th level, but untenable at a higher level.
I would say that this guy is unbalanced vs 6th level. But, is that a bad thing? Does this reflect an "easy path to power" that will come back with a cost at 8th level? 10th level? If the long term costs for this character is steep enough, I would be happy to let a player take this route. There are a few weaknesses in this particular build. Sleep and Daze might give this guy fits while a 6th level character wouldn't be bothered at all. At higher levels, something like Holy Word might be really nasty! Anything that grants Con bonuses does not scale very well with decreased HD. So, for a campaign environment, I will say that this character _might_ be unbalanced. I would need to see the bigger picture to give you my opinion on that though!
Conceptually, I like this build. It would be nice to use as a villian. It would probably give players the fits if they are still using core classes!
