Mods can really ramp up the quest difficulty, and thus the level you need your dialog skills to be at, but if you're not running a mod of that nature (Livonya's or the co8 mods that include it, for example), you won't need too much. What I typically do is get one character with an 18 cha (make him male - or else make sure some male in the party has a decent cha - if you've got a sorcerer, that'll work) and give him 3 levels of rogue. Max out all the dialog skills and his search, open locks, and disable devices, as best you can. With an 18 cha, that'll give him +9 in all five dialog skills and that's enough to do all the quests in the unmodded game. From there on I usually go fighter. F7/R3 is a pretty good build, especially with all the flanking you're able to do in ToEE - the extra +2d6 sneak damage is a major plus. Typically, I make this guy human and give him a decent intelligence. With fighter levels and maybe 14 intelligence, you'll be getting 5 skill points a level even as a fighter, which is enough to keep tumble and one other skill maxed out with another not far behind. All in all, not a bad build, and plenty for the dialog you need in an unmodded game.
Oh, and don't forget to give him the feathered cap.
