How much are you willing to spend and how are you paying for it? If you have the cash then you should buy used so if you do grow tired of the 250/500 you can get rid of it without really losing on the deal. If you dont have the cash then id say go new only becuase used bike payments are killer most of the time. As far as the bike goes I started right into a ZX6-R and ive only been down once, (low speed, odd circumstance kind of thing).
Do yourself a favor and take the MSF class. They usually use Nighthawk 250's and if you take it you will A. Learn some seriously life saving and everyday riding techniques, B. you wont have to take the DMV test for your license and C. you get a good amount of riding time that will help you decide if you want to go with a bike like the nighthawk.