What is the ruling on buying a car from someone for less than the market price, because they are in a hurry to travel and want to sell their belongings at a lower price due to their urgency?

Yes it is permissible; however, a Muslim shouldn't take advantage of other's difficult situations.

I`m infatuated with a man, is it permissible for me to make supplication that he falls in love with me and becomes my husband?

Islam has honored women by being proposed to by men, and not the other way around, so it is inappropriate for a woman to propose to a man since one who hastens in asking for a thing prematurely shall be punished by deprivation. In fact, such a phenomenon is the result of unlawful mixing between the two sexes.

Is it incumbent on the fiancée to obey her fiancé?

When the woman settles in her husband`s house, it is incumbent on him to provide for her and it is incumbent on her to obey him. Before that, and if the marriage contract had been concluded, then she is lawfully his wife and thus she should abide by custom in treating him, but if the marriage contract hadn`t been concluded, then she should treat him as a non-Mahram (Marriageable).

What is the ruling on performing voluntary prayers during official working hours?

All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions

There is no harm if  there was break time during official working hours, but if not then it is impermissible. And Allah Knows Best.