1999 Nissan Maxima oxygen sensors

My service engine soon light recently came on. I had it read and it said it was a post O2 sensor. My mechanic suggested replacing the rear O2 sensor near the catalytic converter. He was mildly concerned regarding some rust developing and suggested in replacing the O2 sensor it might require replacing the catalytic convertor as well due to rust from O2 sensor damaging a fitting which might lead to stripping threads.

My question is- the service engine light just came on within the last couple hundred miles. I drive this car only 3,000-5,000 miles a year. Would there be any concerns if I delayed replacing the oxygen sensor right away. Could not replacing immediately lead to causing more significant repairs down the road because I did not replace the sensor right away. I have not noticed any problems with the performance of the car at this point. (I did replace all of the ignition coils a couple years ago). If you think it is ok to wait awhile, how long (either time or miles) could I comfortably drive the car before I really need to replace the sensor.

My concern is that I have been quoted over $400 to replace the sensor and maybe another $1000 for a convertor if they have problems with the sensor and strip the threads due to the excess rust. They do not expect this to happen but suggest it is a possibility.

Any advice or suggestions would be appreciated.