The O2 sensor can be changed by any exhaust shop in about 15 minutes if you bring the part. Most of the time is waiting for the exhaust system to cool down. Takes a special tool.
Don't buy generic ones or ones without crimped on at the factory pigtails.
Bosch makes the parts for Porsche and they can be had for less without the Porsche label stenciled on. I used two Bosch. More info including part numbers
here.