Is kissing and sucking the vagina considered akin to sexual intercourse in ruling and penalty, and must the one who erred marry the girl with whom he erred?

What occurred between you two is forbidden by Islamic law and necessitates sincere repentance. The Sharia has warned against following the footsteps of Satan, which lead to indecency, including forming a relationship outside the bounds of a lawful marriage. You both must repent to Allah, and He accepts the sincere repentance of His servant, no matter how great their sins. Regret for a sin is considered repentance. What you did is considered a form of zina (fornication/adultery), but it does not reach the level of zina that mandates the prescribed punishment, which is the insertion of the male organ into the female organ. This forbidden relationship must be severed immediately, and if the girl is of good character and religious commitment and is repentant, there is no impediment to proposing to her.