Its a little dark (I suffered similar taking the shots of Ladybower) If your not wanting star trails you need to divide 500 by the focal length, in your case here, 500 divided by 18 = 27, so I'd go with a 25 second exposure.
Then, for every stop you up the ISO it will double the amount of light ie:- a 25 second exposure @ 100 ISO = 25, @ 200 ISO is equivalent to 50 seconds, 400 ISO is equivalent to 100 seconds, ect, ect,
Obviously by opening up the aperture to the max f3.5 would gain you a stop and a third, so 1.3x more light at 25 seconds
Hope that helps, I'd find it a lot easier to show you than I ever could writing it down
Edit
Here's a link explaining how to avoid star trails
I've just realised the 25 seconds @ 18mm is for full frame, you need to divide focal length by 350 on a D7000, scroll down and there's a chart for Nikon 1.5 crop bodies, but the same principle still applies