Here are some things that I would look for if I were buying a budget bike for riding on paved and dirt roads. Relatively light weight bike with a frame made out of aluminum or chromoly steel. That will eliminate the lowest end bikes. Next I would want a 21-speed or higher (3 speeds in front, seven in the back). "Tourney" level components are the lowest end I would accept and would recommend Altus or Acera if you can get it. I would make sure I had indexed shifting (not friction shifting). I would look for a tire that is "32" or "28" with a psi of 80 to 115. I would spin the wheels to see how well they spun around the hub and to make sure there was no wobble in them. I would squeeze the brakes to make sure they grip hard and spring back. Finally, I would sit on it and see if it fit. In addition, I would check with Amazon and Nashbar bikes.