The catalytic converters are a pretty common failure on the 2nd gen Xterra. They have a few failure modes though.
The most common is simply wear. The catalyst gets to where it can't perform its duty. Get a bluetooth OBD dongle from Amazon for about $20 and the Torque app from the Play store. Monitor the down stream O2 sensor voltages. They will be different from side to side and I cannot remember the reason for that at the moment, but what you are looking for is them to be very erratic while the engine is under steady load, like interstate speeds with the cruise control set holding a steady speed.
This is a good indication that your cats are aging and nearing the end of their lives. I think there's a video on Youtube that shows the changes before and after new cats in an Xterra.
There is a second failure mode that's a little more severe. This involves the catalyst material in the converters breaking apart. Initially you'll get the same codes as if the converters are just aging, however in short order, sometimes mere miles from the initial code popping up, the material begins falling out of the case and can plug the secondary cats. At this point, best case scenario is simply bogging down the engine and loss of power. It can in some instances plug it enough for the pressure wave from the exhaust stroke mixed with the EGR function of the cam shafts causes the ceramic catalyst material to enter the cylinders, mainly the rear one on the side that's failing, and just grind the rings and cylinder walls away. That's a death sentence for the engine.
So be leery of running the thing too much before pulling the cats and have a look at them and make sure they aren't coming apart on the inside. The cats aren't very difficult to pull and it's a good idea to get them out and checked.
As for the plugs and O2 sensors, changing them probably wouldn't be a bad idea just due to age. At 180k miles those sensors have been subjected to a lot of heat and vibration and things like that wear like anything else.
