At any point in the game, you can farm abishai (or Undersigil) to gain an unlimited number of levels (and stat points) so that is an option if you're willing to spend the time doing it. Other options include switching to mage and casting a friends spell on yourself, purchasing charisma-raising tattoos from Fell's shop, and completing quests like Sebastion's that raise your charisma. Good luck!
Unlike recent CRPGs such as Pillars of Eternity, Planescape makes an unlimited amount of XP available to you through respawning monsters like Sigil's abishai, UnderSigil's demons, and the Fortress' shadows. So there isn't any great need to worry about maximizing the amount of XP you'll obtain from dialogue.
On the other hand, there is a *huge* amount of lore you'll only learn if your mental stats are sufficiently high. There are also a few things you'll only learn if you are a mage, though this is offset by the fact that mages level very slowly and hence don't give you as many stat points as a thief or fighter Nameless One would have. By endgame, you can easily be losing out on 10+ stat points if you go mage, which is enough to make fighter or (especially) thief worth considering.