Temporary marriage (Mut'ah marriage) is an Islamic marriage which saves those who can not afford permanent marriage. You and her can agree on Mahr and time for the contract. If she is a virgin Muslim girl, her father's permission will be required. She can put a condition on you not to take her virginity until you have permanent marriage.
You can read on this website more details about temporary marriage (Mut'ah marriage) or ask your local Shia mosque scholars about its rules in Shariah.
If you have your family's blessings, and she has her family's blessings, then you can commence with communicating with her, within the shar'i boundaries of course. That would be something positive, to get the know the person prior to the marriage ceremony.
However, if it is away from parents knowing, and without any correct guidance, then you should not do that. A male/female going behind their parents' and secretly communicating with another person, wanting to supposedly marry them, is not at all advisable.
Parents should never be the last to know. That is a very disloyal trait, and in most cases ends very bad. First step is to have family involved, and then everything will fall in place, in shaa Allah.
In our Shari'ah, we do not have any prohibition or censuring of marriage of a Seyid/Seyidah to a non-Seyid/Seyidah. This is a cultural practice and personal preference of a parent.
One must bear in mind though that if one is to marry a Seyid/Seyidah, they must stress on a higher level of honouring and respecting them, due to their lineage to Ahlul Bayt (a.s.).
As this is a frequently asked question, please refer to other answers given on this:
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