Funny thing about the "stadard" undead-creating spells.... most of them are either templates that specifically throw out all class levels (zombie, skeleton), or are defined creatures (Ghoul, Ghast, Mummy, Mohrgh, Shadow, Wraith, Spectre, Devourer) which, while advanceable, aren't necesarily based on the origninal at all ... and none of which specify advancement by character class ... and none of which are listed as potential player monster races, (unlike, say, the Ghost or Vampire, which are not creatable under the standard Create Undead spells ... in the 3.5 SRD, at least; my 3.0 Player's Handbook lists Ghost and Vampire as available under Create Greater Undead, if you have a high enough caster level)