Just don't get something too old. Say more than 5 years. Also, sit in the boat for 30 minutes. That is long enough to tell if your legs will cramp up. You can get a sweat boat for under 500 so don't spend too much. I'd say the most important thing is that YOU like the boat. Most modern creek boats from most major manufacturers are preaty dang good. Wavesport, Dagger, Jackson, Fluid, Pyrahna. That is not a complete list.
How much do you weight and what rivers are you going to get after?