WD21s only have one O2 sensor, which is upstream. Bosch is the original O2 sensor manufacturer. Buy
this Bosch direct-fit sensor at Advance Auto Parts online using discount code
A124 and get it for only $60 before tax.
Loosening and tightening the brass screws on the backside of the instrument cluster may help your tachometer, also ensure the wiring harnesses are connected completely and are "clicked in". If the speedo and tach start working after tightening the screws, it might fix your code 14 too.
KYB GR-2 front struts are great on my '01 Pathfinder, I'm sure they'll be good for your WD21 too.
Does your Pathfinder have a manual transmission?
I don't know anything about emissions testing, Florida doesn't have them and I've never lived in a state with inspections.