Well, you can cast in full plate with no penalty.
Not with Apostle of Peace you can't. Not only can they explicitly not cast in any armor, but they have Vow of Poverty as a prerequisite, so intentionally wearing any armor kills their class features permanently by removing the feat (and also the Mystic Theurge advancement to the arcane side of this build, since he loses the divine spellcasting prerequisite).
As for what to use for the arcane side, part of me leans toward Beguiler, since they have the Whelm line (which deals nonlethal damage), the Hold, Charm, and Dominate lines (which remove people from combat or make them your friends without hurting them), and lots of illusions to confuse or disable enemies. On the other hand, a Wizard or Sorcerer can do those things (maybe not all of them, in the Sorcerer's case, but any of them) and still have ways to deal with constructs and undead (and maybe even plants, oozes, and vermin, if you get creative).