Are you sure you don't want just a single level Cleric dip to get Turn Undead so you can enter the Hunter of the Dead (CA) prestige class?

No? Well... if Complete Psionic is okay with you, why don't you try out the Lurk (one of their augments is Sneak Attack, Undead) or a Soulknife/Illumine Soul, which has a a nifty undead bane permanently applied to his mind blade and positive energy based powers as well as Death Ward?

Otherwise... you could start as something like a Rogue or a Spellthief and use the Penetrating Strike ACF or Razing Strike, both are okay against undead and constructs.