Why is it "helt ny" and not "hel ny"?
"Helt" is an adverb here, and doesn't agree with the car.
Bare hyggelig! :)
Ah, so in other words the 'helt' is referring to the new nature of the car and not specifically the car itself?
I was wondering if I would type "brand new" would it be correct?
Yes, that's one of the accepted translations.