300E O2 Sensor Replacement

Hello everyone.....My neighbors mb/bmw mechanic made a service call nextstore and told me I have to have the O2 Sensor and a catalytic converter hooked up so the car will run without a computetr glitch. After talking to him I looked at the metal end of the 02 sensor and learned that the metal was a beat up and bend a little. I saw that there is a rubber insulation piece that guides the wires through inside somewhere. MY QUESTION- Is there a way to get to the other end from the inside of the car? Is it a hard job? Is 300 dollars a good price for an aftermarket CAT? Whats the difference between one CAT and an aftermarket cat? Thank you all- Charlie- Monmouth County NJ