how harmful is letting a car sit for 8 months?

So i am only 15 but i'm going to be 16 in 8 months. I really want to get a car - my dream car, and to top it all off i might get the opertunity of a life time. An audi tt with 100k miles for only $5,000 and in great condition inside and out! (how could you turn that down?!).

So i've been trying my best to persuade my parents to let me get the car now because it is a 1 time deal with no competition. If i dont take this deal then i wont be able to get that car at all...

So since i dont want to pay for registration and insurance for that long and only being able to take rides in it a few times every week with my dad driving it how bad would it be to let it sit in my driveway and sit it in every night and start her up and possibly rev her a little bit in neutral??

i know its not smart to get a car this early and let it sit there but i just cant bear to pass up the offer!