Negative adjective-noun?

Okay, I'm going to try to keep this short here:

If "This is not old" in Japanese is, これは古くないです, then how would I say, "This is not an old car?" これは古くない車です？　これは古い車じゃない？ Any help would be much appreciated.

both are ok

Saying これは古くない車です is a bit like saying "This is an un-old car." この車は古くないです or これは古い車ではありません definitely sound more natural.

For example, if you are walking through an used car sales lot, and you are showing cars to a potential buyer, pointing out one car after another and telling them about the cars, then (the topic of the sentence would be "this car" so the rest of the sentence would go like:

この車は とても 古いです This car is very old
この車は あまり 古くないです This car is not very old

Thank a bunch for your answer, I couldn't find that ANYWHERE on the net. :D

----これは古い車じゃない？-----

is colloquial for
"..this is not an old car?"

without "?" it would be "this is not an old car."

Yeah, the ? wasn't actually part of the sentence, I was just asking if it was correct or not.

Oh, geez, sorry...looks like I've been sleep talking again.
Truely apologize.
all is correct and AK gave you correct example as well.