There is nothing wrong with getting a 600 as a first bike because you are looking older used and not a new RR. If you drop it or scrape it, it will be cheaper and easier to fix, unless you don't even care how it looks.

I agree with all of the above about you being able to get a good bike for your cash. I also agree with looking for a bike that wasn't used for tricks. Aside from the engine, the clutch takes a beating as does the forks and seals. My bike just past 30,000 miles and I bought it at 28k miles. I've changed all the fluids, chain, sprockets, tires, brakes, and now a battery, but it starts right up and runs great. I wouldn't hesitate to buy another older one if it was cared for before and I can check it out before purchase. Only ways I would get rid of my older CBR is if it got wrecked or I found a nice cheap 954.

 

My F3 just turned 46 K last weekend and it's still running like a raped ape! So, even if a bike has 12 or 15K on the clock... you have plenty of riding left.