What do you do when it outside and there are over 600 cars and 70,000 people?
What DC Fan said. Or, at an evening show, you can wait until some cars have left. The advantage is that you'll have more room & the better "late light". See the "Tips" in my Sig.
Ive been asked to shoot a local car show I'm not sure how its usually done is it done on spec or other wise.
It's whatever both you & they want. Don't expect too much & you won't be disappointed. Most of the car guys here are retired & unlikely to pay a "proper" price for a shot to put on their wall because,
1. They are living on retirement money.
2. They spend all their "extra" money on their cars and,
3. They have a ton of pics already, most of which they got for $10-20.
would a 24-70 lens be all right.
I use a 28-70, so you should be OK. The Kit lens would work, too, but don't get too carried away at the low end. See the framing & angles in my Sig.