A Muslim man (not female) can marry a non-Muslim who is from the people of the book. This includes Christians, Jews and Zoroastrians. Some scholars allow both permanent and temporary marriage with the above mentioned categories, and some only allow temporary marriage with them.
If someone says a Shahada and professes faith, we judge on the apparent and consider them Muslim. If however a Shahada was not taken seriously or said in a joking type of way, it has no value.
So if a person has the intention inside them to say the Shahada just to get married - and this has happened before - it will be accepted if we aren’t aware.
This is similar to hypocrites who utter the Shahada but don’t believe in Islam with their hearts.
If a non Muslim is genuinely willing to accept the faith and marriage is also a motivation for him, it will still be accepted.
May Allah grant you success