What wide band O2 sensor and gauage to buy

LC-1, PLX ?

what sort of price am i expecting to pay.

I have a flashable ECU and cables to flash it.
I have EvoScan to monitor.

do i remove the factory O2 sensor and fit the wide band sensors in it's place, i presume the factory ECU will now use the wide band sensor output and evoscan will monitor it as well.
or do i add the wideband O2 sensor and leave the factory one in place for the ecu to carry on using and the only thing that monitors the wide band sensor is evoscan.
i am not sure how it is configured.