All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
You have indeed made a great mistake by committing that grave major sin, i.e. dealing in Riba (usury). Allaah The Almighty Says (what means): {Allaah destroys interest and gives increase for charities.} [Quran 2:276] Furthermore, the Prophet, sallallaahu ‘alayhi wa salam, said: "May Allaah curse Riba: its consumer, giver, scribe and two witnesses: all are equal (in sin)." [Muslim]
However, to buy a car is not a necessity in so much as it is a need. There is a difference, according to scholars, between necessity and need. Necessity is that without which one is exposed to harm. For this reason, it is permissible for someone who does not find food to eat the flesh of dead animals in order to sustain his life (as he is forced by necessity). In confirmation of that, Allaah The Almighty Says (what means): {But whoever is forced [by necessity], neither desiring [it] nor transgressing [its limit], there is no sin upon him.} [Quran 2:173]
However, no harms befalls a person who does not obtain his need. So, you have to repent to Allaah The Almighty, pray for His forgiveness from this grave major sin, show regret for that, and decide, with determination, not to do it again.
This matter, however, has nothing to do with caring for the orphan, for which you would receive reward in abundance, Allaah willing.
Allaah Knows best.