Shulchan Aruch/Even ha-Ezer/2
A man should make an effort to marry a respectable wife 
(Contains 11 Paragraphs)
1. A man should not marry a women that does not have any kind of inappropriateness. [Isserless: Anyone who marries an inappropriate wife for money will bring them children who are not respectable, but aside from that, if there is no inappropriateness on her, rather he just marries her for money, it is allowed. If someone wants to marry an inappropriate wife, his family members may protest him. If he does not want to be supervised by them, he may make a type of sign such that their offspring will not intermingle with that of his family; and someone who offered a lot of money for him to get married and then took back the offer (even though they got married) he should not hold his wife like a hostage because of this and he should not quarrel with her over her family's property, and someone who does this will not be successful and the relationship will not be a good one because the money that a man takes along with his wife is not appropriate money; and anyone who does this is called someone who "marries for money". Rather, everything that a father or mother in-law gives him should be taken with a good-eye (a full heart) and he will be successful.]
2. Any family with the presumption of a good lineage and he's permitted to marry her a priori. Nevertheless, if you see two families that always irritate one another [Isserless: or two individuals that irritate one another]or you see a family that are upholders of mitzvot and argue always or you see a man that argues disputes with everyone and especially brazen, then we should suspect their lineage and it is fitting to distance oneself from them b/c these are disqualifying characteristics. Similar a person who always invalidates others, for example he puts a stain on the lineage of families or on individuals and he says about them, that that they are mamzerim then we suspect him of being a mamzer. And if he says that they are slaves then we suspect that he is a slave. And all who invalidates others, that blemish (actually) invalidates him. And likewise he who is brazen, cruel, and hates others and he doesn't show them loving kindness we suspect him the most, lest he is gibionite (see Mishnah Torah, Isuri Biya 19:17).
3. When the purity of a families lineage has been disputed, whoever two people testify that there was intermingled with one another, a mamzer or a challal or that they have slaves in their lineage, behold this is doubtful. And if it is a family of priests no one may marry a women from them until they will check about her up through four generations because there are eight involved: her mother, her maternal grandmother, the mother of her maternal grandfather, the maternal grandmother of her maternal grandfather. And likewise we check the mothers of the fathers side: the mother of her paternal grandmother, the mother of her paternal grandfather, and the mother of the mother of her paternal grandfather. And if the family that was called into question is a Levite or Israelite they add to the checking of them another one and we find that they check 10 mothers. But a women that is brought to be married doesn't need to check (her husband's family) because kosher women are not warned against marrying blemished men.
4. Anyone who is called a mamzer or netin or challal or a servant and he is silent about his status, we suspect him and his family and we don't marry them even if we check as was explained. [Isserless: There are some that say this applies only to a family that is known to blend with one of these psulim (inappropriate ones), but an individual that is called thus and keeps quiet, means nothing. There are others who say that all this (from the Mechaber) only applies to earlier generations where Beis Din would keep track of the yichus of families and appropriately apply punishments, so if someone is silent, it can be considered that he admits since he knows that the truth will be revealed. But now, silence is an indication only that he does not want to argue or fight and is thus praiseworthy, so rather today, we accept this as a sign of good yichus as long as he does not cause an argument. Others say that we do not say at all that 'silence is like an admission' and that all this only goes as a sign against the accuser himself and we question his Yichus (not the accused one). Yet, if he is trying to Possel his children in front of him and he remains silent, the silence is never considered and admission (not even by the Mechaber) although, it does raise some concern and reason to check his family. And in all cases, someone who hears troubling words about other matters (aside from Yichus) and he remains silent, this is a clear sign of proper upbringing and that he is appropriate to marry.]
5. A family that might have in their lineage a suspected chalal any kosher woman may marry one of these. And if she is widowed she is forbidden to a Cohen A Priori. But [after the fact] if she is married [to a priest], she doesn't need to be divorced because of these two doubts: Maybe she became a widow of this chalal and maybe she is not a widow. Even if you say that she is a widow maybe he is not chalal. [Isserless: And some hold this is Davka for a the widow herself, since she has a chazaka of Kashrus (having been married to a Kohen) but this does not apply to her daughter and if she is already married, he must divorce her. But, others are Maikel and make no distinction between the widow and her daughter)] But if someone who was certainly a chalal marries into a family, then every women in that family is forbidden (to a priest) until he the priest does an investigation. If she was married (without an investigation), she is then divorced. And this is also the rule if a doubtful or certain mamzer marries into a family. [Isserless: Yet all this is particularly for someone about whom it is known, but a family that is blended with (at least one known) passul that is not known to the public, for example this one or that one was tamei and the knowledge of this passul is not appropriate for public exposure, rather we assume a Hazakah of Kashrut that all the families that become tamei in Israel are Kosher in the days of the Messiah and in every place, this is kosher to reveal and demonstrates their modesty. This is davka for a family that is blended with tameiim, but any time that this tuma is not blended (and the passulim are known) then the tameiim should be revealed and publicly announced about them so that kosher people will keep away from them.]
6. Israelite man should always make an effort to marry a scholar's daughter and to marry his daughter to a scholar. If he can't find a daughter of a scholar he marries the daughter of a gedolei hador (great Rabbi) and he can't find a gedolei hador he marries the daughter of one of the heads of the congregations and if he can't find one of the heads of congregations he marries a collector of Tzedakah and if he he can't find that he marries a school teacher but his daughter should not marry an am haaretz (simpleton). [Isserless: Pertaining to their daughters he says, cursed is the one that lies with beasts and all this pertains to the am haaretz because they are not diligent with the mitzvot. But its a mitzvah for a man to marry the daughter of his sister and there are those who say even the daughter of his brother].
7. A man shouldn't marry a women from a leporous family nor from a nekafim. If a family has three instances the next children will have the presumption of being this way.
8. An am ha'aretz should not marry a cohen. And if he marries one, their pairing will not develop well (Pesachim 49a) because she will die or it (meaning the child) will die from the pregnancy or problems will come between them. But a scholar that marries an Cohen woman, this is pleasant and praising of Torah and as it was discussed in the first place.
9. A young man should not marry an old woman and an old man should not marry a young girl for this thing causes whoredom.
10. A man should not marry a woman with the intention of divorcing her and if he announces from the beginning that he is going to marry her for a few days then that is permitted.
11. A man should not marry a woman in one land and marry another woman in a different land lest the children of this one and that one (meaning, of each woman) pair off with each other and we find that a brother marries his sister but a famous person and his children are well known because of him, then it is permitted (to marry women in different lands).