All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
What you have claimed, that the person can make whatever mistakes he wants and then repent, and that his sins would simply be removed and he would then be held in the same status as a person who has not committed any sin to begin with, is incorrect and untrue. The Muslim is enjoined to abide by the Islamic sharee'ah and avoid all sins, minor and major alike.
You should know that having the audacity to commit sins under the pretext that one can simply repent of them later on is the work of the Devil; verily, no one knows when he will die. Moreover, tasting the allure of sinful acts leads to getting used to committing them and, accordingly, it becomes rather difficult for the person to develop a sense of self-restraint against committing sins in the future. Committing a sin often leads to further indulgence in evil and sinful deeds. None is safe from such a peril except he whom Allaah has guided to sincere repentance, which entails extensive exercise of self-restraint to give up such a sin. This hard work and striving in exercising self-restraint and giving up the sin after tasting the allure of sinful acts surpass the efforts of a person who has not committed such a sin to begin with. Moreover, the repentant should display diligence and strive in worship and in carrying out acts of obedience.
People once disputed as to whom is better in the sight of Allaah, the one who has not committed the sin or the one who has committed it and then repented. Ibn Al-Qayyim asserted that this matter varies depending on the sincerity of one's repentance and his diligence in worship afterwards. Perhaps the repentant keenly strives in carrying out acts of worship and obedience and accordingly can outrun the person who has not sinned. Moreover, the one who has not committed sins may also outrun the repentant if he proved to be keener in his striving in worship.
In general, the Muslim is enjoined to avoid sins, major or minor; he should not boldly commit misdeeds under the pretext that he will repent of them later on; verily, this is the work of the Devil. When one commits a sin, he should hasten to sincere repentance to have the sin removed (and earn the forgiveness of his Lord). He should also strive in carrying out good deeds so as to erase his sin; Allaah, The Exalted, says (what means): {Indeed, good deeds do away with misdeeds.} [Quran 11:114]
This is as far as the first and third parts of your question are concerned.
As for this suitor who proposed to marry you, he did wrong by informing you of his past sin. He should have concealed his sin since Allaah has concealed it for him so far; he should not have informed anyone of it. Please refer to fatwa 84052.
As for your question as to whether you should marry him or not, we advise you to consider his current state only and see how steadfast he is on the straight path now; if he is pious and well-mannered and sincerely repented to Allaah of his past sins, which would be indicated by the manifest signs of righteousness and goodness, then it is advisable that you should not refuse his marriage proposal. Accepting his proposal in this case does not constitute injustice to you nor does it imply undermining your status in the slightest given that you have not committed such a sin yourself. The decisive factor in evaluating someone's character should be his current state only, and not his past. You may marry a righteous person who has not committed such a sin, and he might commit it after your marriage. Hence, the criterion in evaluating a person should be his current state and how steadfast he is on the straight path, and not his past.
Finally, Allaah, The Exalted, is Just; He never wrongs anyone in the least. Perhaps that repentant man has endured great hardship in the course of his repentance, giving up sins, and striving in exercising self-reproach and self-restraint, entailing him to earn greater rewards compared to someone who has not committed such a sin to begin with. So have confidence in the justice and wisdom of your Lord and preoccupy yourself with whatever reinforces your steadfastness and compliance with the sharee'ah and Islamic teachings, avoidance of the sins and wrongdoing, and turning to Allaah in sincere repentance.
Allaah knows best.