I’ve purchased mine with 150K miles and kicking strong 
It threw P0420 around 400 after purchase
As I checked cats, the upstream cat on bank-2 was already replaced before me, the one on back-1 was rusty as hell, so I wen to replace the upstream cat in bank-1 with Shultz (eBay, $130), code disappeared
my next entertainment was with PA200, which it threw 200 miles down the road, giving it to next long and windy story: Bank 1 runs "lean" after upstream oxygen sensor replacement
In essence, it was seeing oxygen in exhaust, which came from the leak on upstream cat and on flange between an upstream cat and downstream cat
at this point, I replaced [leaking] Shults by Pacesetter, which was much more solid in welds and had no leaks, had to replace downstream cat as its front flange disintegrated
Pacesetter is 50K miles warranty on “structural integrity” (stainless steel!) and 25K warranty on cat performance: pretty much the best you get for under $200 cat
by that point I was so frustrated, I wanted to have full assurance I’m not getting back to emission system for next several years, so just to be sure I’m done with this, I’ve replaced both banks downstream sensors using these: http://www.ebay.com/itm/252640813337?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
If you buy these: you will need to use a sharp knife to cut a “key” on one of sensors to fit into connector on vehicle, as these stupid guys make left/right sensors to be different, only to accommodate for 2 inches difference in wire length
I’ve also replaced both upstream Fuel/Air Ratio sensors with OEM (BOSCH): https://www.amazon.com/gp/product/B000BZJA4Q/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
the rationale to use cheap downstream sensors and good expensive A/F sensors for me was to make sure that A/F is properly regulated (the most important for CAT longevity), and than I did not care much about downstream O2 ones, as their performance is less critical overall, so I skimp on that part
drove 2000+ miles after repairs - ALL GOOD
you will indeed need your P0420 / P0430 repaired or you will fail emissions
working on cats in R51 is a complete disaster:
- exhaust heat-shields have 6mm bolts you will most likely break, unless you use some rust penetrating oil overnight
- althrouh car is not small, it is very tight space, especially on passenger side - I needed to remove engine hook-eye and move wire cable aside to slide heatshield there before I could do anything
- at this point you will benefit from S-shaped wrenches you can find in Harbor Freight to have better grip on nuts
- flange on the downstream cat after 10 years pretty much rusts out (fell to my hands on second disassembly), so you assemble everything only to discover a leak on the flange and that stupid P2A00 code, save yourself time, buy downstream cat[s] from Pacesetter too - they are great quality and made from stainless teel
- all downstream cat bolts/nuts were rusted out to the point I needed to use grinder to cut them off
- I’ve also discovered that my PCV valve was not a valve anymore - it was not holding well and engine was sucking oil into the intake, probably accelerating CATs wear
if I knew everything from the beginning, I would probably make everything work all by myself over one weekend, but since it was unfolding as a “thread from the sweater”, I was mad by the point it ended
you can definitely do it on your own and save a lot $$$, as in the end it did not require any super-special tool, mostly persistence and some perspiration
I would recommend you replacing sensors too, as likely yours are on their limp
while sensors can hold longer, they can be considered up to replacement at around 100K miles
if you replace sensors - go to nissanhelp.com to get recalibration procedures done (A/F ration self-learn, idle air self-learn)
your spark plugs are also due at 100K, BTW
I’ve mentioned a PCV valve to be replaced above: cheap, but important to do as it can let oil to be burned excessively, poisoning your new cats
another thing I might mention: in my Pathfinder, it was a smell of burnt oil coming from exhaust right after start, then disappearing as it heat up
as I replaced spark plugs - they had some oil residue, so I decided to use “Marvel Mystery Oil” (under $4, Walmart) to clean my engine, by adding to oil, that smell after startup disappeared in 150-200 miles, and I did not yet replace PCV at that point, so it was definitely that I had some sticking of oil-removing piston rings, recognized on early state and it helped
good luck