I hope to fly into JFK airport at the start of July this year.  I have
found that I will need to hire a car for around 3 weeks, that should
not be an issue if it wasn't for my age.  I am currently 18 and to
hire a car it costs me almost twice as much as it would for someone
over the age of 25, for example Hertz are charging me an addtional $51
for a car that would normal cost $80 a day.

I am considering buying a second hand car once I get to NYC and sell
it before I leave.  What procedures need to be carried out for buying
a car in NYC, does it need to be registered and insured?  Is this the
case in all the states?  Would it be wiser just to rent the car?

Thank you