Beguiler practically does everything you want all on its own, so you probably want that on one side. For gestalt combos:
Beguiler//Factotum: Very strong int-synergy, has trouble just pushing out damage when needed, but with your party sounds like that's covered fine. Limit yourself to one extra standard action per turn, or the DM WILL likely kill your character off...
Beguiler//Warblade: Not as much int-synergy, but still a lot, and much beefier combat stats (full BAB, d12 HD, good fort saves) at the expense of every skill as a class skill and some other utility. Maneuvers give you powerful melee options, though you can't use most of them while casting. This is probably the most solid choice all-round.
Beguiler//Duskblade: d8 HD, full BAB, good fort save, pretty solid overall. Also int-based casting and can cast in armor. You can't use spellcasting from both the same round (barring quickened spells), but duskblade's normally limited spell list covers much of the stuff Beguiler is missing, and has a NASTY full attack. I'd need to check, but you could probably use Quick Cast on beguiler spells, which would be very powerful if so.