[KimMG]

Are you required to pass an emissions test in your area? Is there an O2 sensor after the catalytic converter? If yes, then the computer will get the wrong readings and may cause the car to run rich or lean. Take it to a muffler shop and see what they have to say about non oem replacement. Take the car to Autozone and have the codes read for yourself.
If you are sure that this is the car you want to buy, then pay for an inspection. The more you know, the fewer surprises and disappointments you will have.