How difficult is it to change out O2 sensors and how long should it take? I have a 1998 Disco. also, how effective are the universal sensors versus the $170 ones. Thanks for your time

 

They are easy to replace, based on your mechanical exprience.
Second, you can buy non universal 02 sensors for around $65 a piece, so don't cut corners or you will get what you paid for, junk sensors.
Go to Atlantic British on line or Auto Zone, you should be able to find a much better price then what you mentioned.