Praise be to Allah.
First of all, may Allaah reward you with good for your kind words. We apologize for not answering questions in full. With regard to your question about this man who has asked for your hand in marriage, what you should concentrate on is the way he is now: is he doing his religious duties, like the five daily prayers, etc., is he keeping away from haraam things, has he repented for what he did in the past? Or not? If he is keeping his duty towards Allaah, then this is the good commitment to religion that is required according to the command of the Prophet (peace and blessings of Allaah be upon him): If there comes to you with an offer of marriage (for your daughter, sister, etc.) one with whose religious commitment and character you are pleased, then marry (your daughter, sister, etc, to) him. If you do not do so, there will be mischief on earth and widespread corruption.(Reported by al-Tirmidhi, 1004; classed as hasan in Saheeh al-Jaami, 270)
It is not permitted to bring up the past of a person who regrets it and has repented and given up his sin; rather, this should be covered up. Whoever covers up his Muslim brother in this world, Allaah will cover him up in the Hereafter.(A saheeh hadeeth reported by Imaam Ahmad; Saheeh al-Jaami, 6287)
But if a person is immoral and corrupt, and still has relationships from before, then you should never, ever agree to marry him. Allaah says (interpretation of the meaning): The adulterer marries not but an adulteress or a mushrikah. And the woman who agrees to marry a mushrik or an adulterer, then she is either a prostitute or a mushrikah. Such a thing is forbidden to the believers. [al-Noor 24:3]
Ibn Katheer said, commenting on the phrase Such a thing is forbidden to the believers: i.e., (it is forbidden for) a man to marry an immoral woman, or for a chaste woman to marry an immoral man. Hence Imaam Ahmad ibn Hanbal said that the marriage contract between a chaste man and an immoral woman is invalid, so long as she remains like this, unless she is asked to repent and she does so, in which case the contract becomes valid, otherwise it is not valid. Similarly, the marriage of a free, chaste woman to a promiscuous, immoral man is invalid unless he repents sincerely, because Allaah says Such a thing is forbidden to the believers.
There is no secret about the mischief, misery and problems that result from marriage to an immoral man.
In many cases it is very difficult to get a true picture of a person and be sure about whether he is chaste or otherwise. But by researching, asking questions, consulting people and asking them for advice, whilst taking ones time and asking Allaah for help, one can get answers. We ask Allaah to choose the best for you, to help you and to guide you. May Allaah bless our Prophet Muhammad.