The World's Largest Dungeon
The sorcerer is a good choice for arcane support in this campaign (which is recommended). The benefits of the sorcerer are that spells are awakened or discovered within the PC which means as the PC levels they gain new spells just as they would outside the dungeon. The trade-off to a sorcerer is of course that once a spell choice is made, depsite the ability to choose what they cast when they need to they are limited to the spells they have chosen to have awoken within them without the aid of scrolls.
Any sorcerer PC may start play with a familiar per the rules. If the player decides to wait to call for a more powerful familiar at later levels they risk not getting it due to being in the dungeon.