Epic Magic is easily broken. Sufficiently so that your DM probably doesn't deserve that kind of punishment. One house rule we adopted was to constrain people more or less to the ones already listed in the book.
I think Ultimate Magus isn't a bad idea. Neither is, of course, Incantatrix -- taking control of other people's spell effects is fun. It might be more helpful if you told us more about the kind of character you want to play. You mentioned TreantMonk's guide, so are you thinking God Wizard? Also, I'd be more than a little concerned with you utterly overshadowing the Monk and Barbarian.
One trouble you're going to run into is that high epic monsters just have numbers that are absurd. So, don't back off having a caster level of ... whatever you can get so that you can laugh at SR and so on. For what it's worth, there aren't really many CR 45ish monsters printed by WotC, so he may have to be throwing templates (and a paragon whatever is going to have dumb saves, for instance), or content himself with lower CRs.
Off the top of my head, and going with the God Wizard approach, I'd be tempted towards Incantatrix and Initiate of the 7Fold Veil. I am not the biggest Iot7V fan in the world, but the defense at super high levels, especially as an immediate action, may be really helpful. I'd also make sure to get some way to pump my CL up to the stratosphere -- probably Circle Magic.
As to Sorc/Wiz, Ultimate Magus is a great suggestion, and maybe a worthy alternative to Incantatrix for some free metamagics. That being said, though, I think there's no use to adding Sorc spellcasting to a super high level Wizard. It's not like you're going to run out of spells or anything.
Given that most of your party is melee, I'd probably go the super ultra buffing route, too. It's a way of playing God, and one that's hard to interfere with. You might need Spellguard of Silverymoon to get some of the great personal buffs onto your allies, and then turn the Barbarian and the Monk into engines of destruction. Hell, I'd probably throw War Weaver on there, too. It's not like you have to worry about losing a few caster levels! And, the War Weaver's action economy is boss, although I guess Time Stop is always there to fall back on.
Finally, I'd suggest picking up Master Staff if you can and then a crazy powerful Epic Staff for blasting. Sometimes blasting is just fun, and it's very Epic Wizardy, and this is a cheap way to get a ton of it.